Playing with numbers Exercise 3.1
Q 1. Write all the factors of the following numbers:
(a) 24
(b) 15
(c) 21
(d) 27
(e) 12
(f) 20
(g) 18
(h) 23
(i) 36
Answer:
(a) 24
24 = 1 × 24 24 = 2 × 12 24 = 3 × 8
24 = 4 × 6 24 = 6 × 4
∴Factors of 24 are 1, 2, 3, 4, 6, 8, 12, and 24
(b) 15
15 = 1 × 15 15 = 3 × 5 15 = 5 × 3
∴factors of 15 are 1, 3, 5, and 15
(c) 21
21 = 1 × 21 21 = 3 × 7 21 = 7 × 3
∴Factors of 21 are 1, 3, 7, and 21
(d) 27
27 = 1 × 27 27 = 3 × 9 27 = 9 × 3
∴Factors of 27 are 1, 3, 9, and 27
(e) 12
12 = 1 × 12 12 = 2 × 6 12 = 3 × 4 12 = 4 × 3
∴Factors of 12 are 1, 2, 3, 4, 6, and 12
(f) 20
20 = 1 × 20 20 = 2 × 10 20 = 4 × 5 20 = 5 × 4
∴Factors of 20 are 1, 2, 4, 5, 10, and 20
(g) 18
18 = 1 × 18 18 = 2 × 9 18 = 3 × 6 18 = 6 × 3
∴Factors of 18 are 1, 2, 3, 6, 9, and 18
(h) 23
23 = 1 × 23 23 = 23 × 1
∴ Factors of 23 are 1 and 23
(i) 36
36 = 1 × 36 36 = 2 × 18 36 = 3 × 12 36 = 4 × 9
36 = 6 × 6
∴Factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, and 36
Q. 2 Write first five multiplies of:
(a) 5 (b) 8 (c) 9
Answer :
(a) 5 × 1 = 5 5 × 2 = 10 5 × 3 = 15 5 × 4 = 20 5 × 5 = 25
∴ The required multiples are 5, 10, 15, 20, and 25.
(b) 8 × 1 = 8 8 × 2 = 16 8 × 3 = 24 8 × 4 = 32 8 × 5 = 40
∴ The required multiples are 8, 16, 24, 32, and 40.
(c) 9 × 1 = 9 9 × 2 = 18 9 × 3 = 27 9 × 4 = 36 9 × 5 = 45
∴ The required multiples are 9, 18, 27, 36, and 45.
Q. 3 Match the items in column 1 with the items in column 2.
Column 1 Column 2
(i) 35 (a) Multiple of 8
(ii) 15 (b) Multiple of 7
(iii) 16 (c) Multiple of 70
(iv) 20 (d) Factor of 30
(v) 25 (e) Factor of 50
(f) Factor of 20
Answer :
Column 1 Column 2
(i) 35 (b) Multiple of 7
(ii) 15 (d) Factor of 30
(iii) 16 (a) Multiple of 8
(iv) 20 (f) Factor of 20
(v) 25 (e) Factor of 50
Q.4 Find all the multiples of 9 up to 100.
Answer :
9 × 1 = 9 9 × 2 = 18 9 × 3 = 27 9 × 4 = 36 9 × 5 = 45
9 × 6 = 54 9 × 7 = 63 9 × 8 = 72 9 × 9 = 81 9 × 10 = 90
9 × 11 = 99
Therefore, the multiples of 9 up to 100 are
9, 18, 27, 36, 45, 54, 63, 72, 81, 90, and 99
Playing with numbers Exercise 3.2
Q. 1 What is the sum of any two (a) Odd numbers? (b) Even numbers?
Answer :
(a) The sum of two odd numbers is even.
e.g., 1 + 3 = 4
13 + 19 = 32
(b) The sum of two even numbers is even.
e.g., 2 + 4 = 6
10 + 18 = 28
Q. 2 State whether the following statements are True or False:
(a) The sum of three odd numbers is even.
(b) The sum of two odd numbers and one even number is even.
(c) The product of three odd numbers is odd.
(d) If an even number is divided by 2, the quotient is always odd.
(e) All prime numbers are odd.
(f) Prime numbers do not have any factors.
(g) Sum of two prime numbers is always even.
(h) 2 is the only even prime number.
(i) All even numbers are composite numbers.
(j) The product of two even numbers is always even.
Answer :
(a) False 3 + 5 + 7 = 15, i.e., odd
(b) True 3 + 5 + 6 = 14, i.e., even
(c) True 3 × 5 × 7 = 105, i.e., odd
(d) False 4 ÷ 2 = 2, i.e., even
(e) False 2 is a prime number and it is also even
(f) False 1 and the number itself are factors of the number
(g) False 2 + 3 = 5, i.e., odd
(h) True
(i) False 2 is a prime number
(j) True 2 × 4 = 8, i.e., even
Q. 3 The numbers 13 and 31 are prime numbers. Both these numbers have same digits 1 and 3. Find such pairs of prime numbers up to 100.
Answer :
17, 71
37, 73
79, 97
Q. 4 Write down separately the prime and composite numbers less than 20.
Answer :
Prime numbers less than 20 are
2, 3, 5, 7, 11, 13, 17, 19
Composite numbers less than 20 are
4, 6, 8, 9, 10, 12, 14, 15, 16, 18
Q. 5 What is the greatest prime number between 1 and 10?
Answer :
Prime numbers between 1 and 10 are 2, 3, 5, and 7. Among these numbers, 7 is the greatest.
Q. 6 Express the following as the sum of two odd primes.
(a) 44 (b) 36 (c) 24 (d) 18
Answer :
(a) 44 = 37 + 7
(b) 36 = 31 + 5
(c) 24 = 19 + 5
(d) 18 = 11 + 7
Q. 7 Give three pairs of prime numbers whose difference is 2.
[Remark: Two prime numbers whose difference is 2 are called twin primes].
Answer:
3, 5
41, 43
71, 73
Q. 8 Which of the following numbers are prime?
(a) 23 (b) 51 (c) 37 (d) 26
Answer:
(a) 23 23 = 1 × 23 23 = 23 × 1
23 has only two factors, 1 and 23. Therefore, it is a prime number.
(b) 51 51 = 1 × 51 51 = 3 × 17
51 has four factors, 1, 3, 17, 51. Therefore, it is not a prime number. It is a composite number.
(c) 37
It has only two factors, 1 and 37. Therefore, it is a prime number.
(d) 26
26 has four factors (1, 2, 13, 26). Therefore, it is not a prime number. It is a composite number.
Q. 9 Write seven consecutive composite numbers less than 100 so that there is no prime number between them.
Answer :
Between 89 and 97, both of which are prime numbers, there are 7 composite numbers. They are
90, 91, 92, 93, 94, 95, 96
Numbers Factors
90 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
91 1, 7, 13, 91
92 1, 2, 4, 23, 46, 92
93 1, 3, 31, 93
94 1, 2, 47, 94
95 1, 5, 19, 95
96 1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 96
Q. 10 Express each of the following numbers as the sum of three odd primes:
(a) 21 (b) 31 (c) 53 (d) 61
Answer:
(a) 21 = 3 + 7 + 11
(b) 31 = 5 + 7 + 19
(c) 53 = 3 + 19 + 31
(d) 61 = 11 + 19 + 31
Q. 11 Write five pairs of prime numbers less than 20 whose sum is divisible by 5.
(Hint: 3 + 7 = 10)
Answer:
2 + 3 = 5
2 + 13 = 15
3 + 17 = 20
7 + 13 = 20
19 + 11 = 30
Q. 12 Fill in the blanks:
(a) A number which has only two factors is called a _______.
(b) A number which has more than two factors is called a _______.
(c) 1 is neither _______ nor _______.
(d) The smallest prime number is _______.
(e) The smallest composite number is _______.
(f) The smallest even number is _______.
Answer:
(a) Prime number
(b) Composite number
(c) Prime number, composite number
(d) 2
(e) 4
(f) 2
Playing with numbers Exercise 3.3
Q. 1 Using divisibility tests, determine which of the following numbers are divisible by 2; by 3; by 4; by 5; by 6; by 8; by 9; by 10; by 11 (say, yes or no):
Answer:
Q. 2: Using divisibility tests, determine which of the following numbers are divisible by 4; by 8:
(a) 572 (b) 726352 (c) 5500 (d) 6000
(e) 12159 (f) 14560 (g) 21084 (h) 31795072
(i) 1700 (j) 2150
Answer :
(a) 572
The last two digits are 72. Since 72 is divisible by 4, the given number is also divisible by 4.
The last three digits are 572. Since 572 is not divisible by 8, the given number is also not divisible by 8.
(b) 726352
The last two digits are 52. As 52 is divisible by 4, the given number is also divisible by 4.
The last three digits are 352. Since 352 is divisible by 8, the given number is also divisible by 8.
(c) 5500
Since last two digits are 00, it is divisible by 4.
The last 3 digits are 500. Since 500 is not divisible by 8, the given number is also not divisible by 8.
(d) 6000
Since the last 2 digits are 00, the given number is divisible by 4.
Since the last 3 digits are 000, the given number is divisible by 8.
(e) 12159
The last 2 digits are 59. Since 59 is not divisible by 4, the given number is also not divisible by 4.
The last 3 digits are 159. Since 159 is not divisible by 8, the given number is not divisible by 8.
(f) 14560
The last two digits are 60. Since 60 is divisible by 4, the given number is divisible by 4.
The last 3 digits are 560. Since 560 is divisible by 8, the given number is divisible by 8.
(g) 21084
The last two digits are 84. Since 84 is divisible by 4, the given number is divisible by 4.
The last three digits are 084. Since 084 is not divisible by 8, the given number is not divisible by 8.
(h) 31795072
The last two digits are 72. Since 72 is divisible by 4, the given number is divisible by 4.
The last three digits are 072. Since 072 is divisible by 8, the given number is divisible by 8.
(i) 1700
The last two digits are 00. Since 00 is divisible by 4, the given number is divisible by 4.
The last three digits are 700. Since 700 is not divisible by 8, the given number is not divisible by 8.
(j) 2150
The last two digits are 50. Since 50 is not divisible by 4, the given number is not divisible by 4.
The last three digits are 150. Since 150 is not divisible by 8, the given number is not divisible by 8.
Q.3: Using divisibility tests, determine which of following numbers are divisible by 6:
(a) 297144 (b) 1258 (c) 4335 (d) 61233
(e) 901352 (f) 438750 (g) 1790184 (h) 12583
(i) 639210 (j) 17852
Answer:
(a) 297144
Since the last digit of the number is 4, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 27. Since 27 is divisible by 3, the given number is also divisible by 3.
As the number is divisible by both 2 and 3, it is divisible by 6.
(b) 1258
Since the last digit of the number is 8, it is divisible by 2 On adding all the digits of the number, the sum obtained is 16. Since 16 is not divisible by 3, the given number is also not divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
(c) 4335
The last digit of the number is 5, which is not divisible by 2. Therefore, the given number is also not divisible by 2.
On adding all the digits of the number, the sum obtained is 15. Since 15 is divisible by 3, the given number is also divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
(d) 61233
The last digit of the number is 3, which is not divisible by 2. Therefore, the given number is also not divisible by 2.
On adding all the digits of the number, the sum obtained is 15. Since 15 is divisible by 3, the given number is also divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
(e) 901352
Since the last digit of the number is 2, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 20. Since 20 is not divisible by 3, the given number is also not divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
(f) 438750
Since the last digit of the number is 0, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 27. Since 27 is divisible by 3, the given number is also divisible by 3.
As the number is divisible by both 2 and 3, it is divisible by 6.
(g) 1790184
Since the last digit of the number is 4, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 30. Since 30 is divisible by 3, the given number is also divisible by 3.
As the number is divisible by both 2 and 3, it is divisible by 6.
(h) 12583
Since the last digit of the number is 3, it is not divisible by 2.
On adding all the digits of the number, the sum obtained is 19. Since 19 is not divisible by 3, the given number is also not divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
(i) 639210
Since the last digit of the number is 0, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 21. Since 21 is divisible by 3, the given number is also divisible by 3.
As the number is divisible by both 2 and 3, it is divisible by 6.
(j) 17852
Since the last digit of the number is 2, it is divisible by 2.
On adding all the digits of the number, the sum obtained is 23. Since 23 is not divisible by 3, the given number is also not divisible by 3.
As the number is not divisible by both 2 and 3, it is not divisible by 6.
Q 4: Using divisibility tests, determine which of the following numbers are divisible by 11:
(a) 5445
(b) 10824
(c) 7138965
(d) 70169308
(e) 10000001
(f) 901153
Answer :
(a) 5445
Sum of the digits at odd places = 5 + 4 = 9
Sum of the digits at even places = 4 + 5 = 9
Difference = 9 − 9 = 0
As the difference between the sum of the digits at odd places and the sum of the digits at even places is 0, therefore, 5445 is divisible by 11.
(b) 10824
Sum of the digits at odd places = 4 + 8 + 1 = 13
Sum of the digits at even places = 2 + 0 = 2
Difference = 13 − 2 = 11
The difference between the sum of the digits at odd places and the sum of the digits at even places is 11, which is divisible by 11. Therefore, 10824 is divisible by 11.
(c) 7138965
Sum of the digits at odd places = 5 + 9 + 3 + 7 = 24
Sum of the digits at even places = 6 + 8 + 1 = 15
Difference = 24 − 15 = 9
The difference between the sum of the digits at odd places and the sum of digits at even places is 9, which is not divisible by 11. Therefore, 7138965 is not divisible by 11.
(d) 70169308
Sum of the digits at odd places = 8 + 3 + 6 + 0 = 17
Sum of the digits at even places = 0 + 9 + 1 + 7 = 17
Difference = 17 − 17 = 0
As the difference between the sum of the digits at odd places and the sum of the digits at even places is 0, therefore, 70169308 is divisible by 11.
(e) 10000001
Sum of the digits at odd places = 1
Sum of the digits at even places = 1
Difference = 1 − 1 = 0
As the difference between the sum of the digits at odd places and the sum of the digits at even places is 0, therefore, 10000001 is divisible by 11.
(f) 901153
Sum of the digits at odd places = 3 + 1 + 0 = 4
Sum of the digits at even places = 5 + 1 + 9 = 15
Difference = 15 − 4 = 11
The difference between the sum of the digits at odd places and the sum of the digits at even places is 11, which is divisible by 11. Therefore, 901153 is divisible by 11.
Q. 5: Write the smallest digit and the greatest digit in the blank space of each of the following numbers so that the number formed is divisible by 3:
(a) ___6724
(b) 4765 ___2
Answer :
(a) _6724
Sum of the remaining digits = 19
To make the number divisible by 3, the sum of its digits should be divisible by 3.
The smallest multiple of 3 which comes after 19 is 21.
Therefore, smallest number = 21 − 19 = 2
Now, 2 + 3 + 3 = 8
However, 2 + 3 + 3 + 3 = 11
If we put 8, then the sum of the digits will be 27 and as 27 is divisible by 3, the number will also be divisible by 3.
Therefore, the largest number is 8.
(b) 4765_2
Sum of the remaining digits = 24
To make the number divisible by 3, the sum of its digits should be divisible by 3. As 24 is already divisible by 3, the smallest number that can be placed here is 0.
Now, 0 + 3 = 3
3 + 3 = 6
3 + 3 + 3 = 9
However, 3 + 3 + 3 + 3 = 12
If we put 9, then the sum of the digits will be 33 and as 33 is divisible by 3, the number will also be divisible by 3.
Therefore, the largest number is 9.
Q. 6: Write a digit in the blank space of each of the following numbers so that the number formed is divisible by 11:
(a) 92 ___ 389
(b) 8 ___9484
Answer :
(a) 92_389
Let a be placed in the blank.
Sum of the digits at odd places = 9 + 3 + 2 = 14
Sum of the digits at even places = 8 + a + 9 = 17 + a
Difference = 17 + a − 14 = 3 + a
For a number to be divisible by 11, this difference should be zero or a multiple of 11.
If 3 + a = 0, then
a = − 3
However, it cannot be negative.
A closest multiple of 11, which is near to 3, has to be taken. It is 11itself.
3 + a = 11
a = 8
Therefore, the required digit is 8.
(b) 8_9484
Let a be placed in the blank.
Sum of the digits at odd places = 4 + 4 + a = 8 + a
Sum of the digits at even places = 8 + 9 + 8 = 25
Difference = 25 − (8 + a)
= 17 − a
For a number to be divisible by 11, this difference should be zero or a multiple of 11.
If 17 − a = 0, then
a = 17
This is not possible.
A multiple of 11 has to be taken. Taking 11, we obtain
17 − a = 11
a = 6
Therefore, the required digit is 6.
Playing with numbers Exercise 3.4
Q. 1: Find the common factors of:
(a) 20 and 28
(b) 15 and 25
(c) 35 and 50
(d) 56 and 120
Answer :
(a) Factors of 20 = 1, 2, 4, 5, 10, 20
Factors of 28 = 1, 2, 4, 7, 14, 28
Common factors = 1, 2, 4
(b) Factors of 15 = 1, 3, 5, 15
Factors of 25 = 1, 5, 25
Common factors = 1, 5
(c) Factors of 35 = 1, 5, 7, 35
Factors of 50 = 1, 2, 5, 10, 25, 50
Common factors = 1, 5
(d) Factors of 56 = 1, 2, 4, 7, 8, 14, 28, 56
Factors of 120 = 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120
Common factors = 1, 2, 4, 8
Q. 2: Find the common factors of:
(a) 4, 8 and 12
(b) 5, 15 and 25
Answer :
(a) 4, 8, 12
Factors of 4 = 1, 2, 4
Factors of 8 = 1, 2, 4, 8
Factors of 12 = 1, 2, 3, 4, 6, 12
Common factors = 1, 2, 4
(b) 5, 15, and 25
Factors of 5 = 1, 5
Factors of 15 = 1, 3, 5, 15
Factors of 25 = 1, 5, 25
Common factors = 1, 5
Q. 3: Find first three common multiples of:
(a) 6 and 8
(b) 12 and 18
Answer :
(a) 6 and 8
Multiple of 6 = 6, 12, 18, 24, 30…..
Multiple of 8 = 8, 16, 24, 32……
3 common multiples = 24, 48, 72
(b) 12 and 18
Multiples of 12 = 12, 24, 36, 48
Multiples of 18 = 18, 36, 54, 72
3 common multiples = 36, 72, 108
Q. 4:Write all the numbers less than 100 which are common multiples of 3 and 4.
Answer :
Multiples of 3 = 3, 6, 9, 12, 15…
Multiples of 4 = 4, 8, 12, 16, 20…
Common multiples = 12, 24, 36, 48, 60, 72, 84, 96
Q. 5: Which of the following numbers are co-prime?
(a) 18 and 35
(b) 15 and 37
(c) 30 and 415
(d) 17 and 68
(e) 216 and 215
(f) 81 and 16
Answer :
(a) 18 and 35
Factors of 18 = 1, 2, 3, 6, 9, 18
Factors of 35 = 1, 5, 7, 35
Common factor = 1
Therefore, the given two numbers are co-prime.
(b) 15 and 37
Factors of 15 = 1, 3, 5, 15
Factors of 37 = 1, 37
Common factors = 1
Therefore, the given two numbers are co-prime.
(c) 30 and 415
Factors of 30 = 1, 2, 3, 5, 6, 10, 15, 30
Factors of 415 = 1, 5, 83, 415
Common factors = 1, 5
As these numbers have a common factor other than 1, the given two numbers are not co-prime.
(d) 17 and 68
Factors of 17 = 1, 17
Factors of 68 = 1, 2, 4, 17, 34, 68
Common factors = 1, 17
As these numbers have a common factor other than 1, the given two numbers are not co-prime.
(e) 216 and 215
Factors of 216 = 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 27, 36, 54, 72, 108, 216
Factors of 215 = 1, 5, 43, 215
Common factors = 1
Therefore, the given two numbers are co-prime.
(f) 81 and 16
Factors of 81 = 1, 3, 9, 27, 81
Factors of 16 = 1, 2, 4, 8, 16
Common factors = 1
Therefore, the given two numbers are co- prime.
Q. 6: A number is divisible by both 5 and 12. By which other number will that number be always divisible?
Answer :
Factors of 5 = 1, 5
Factors of 12 = 1, 2, 3, 4, 6, 12
As the common factor of these numbers is 1, the given two numbers are co- prime and the number will also be divisible by their product, i.e. 60, and the factors of 60, i.e., 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60.
Q.7: A number is divisible by 12. By what other number will that number be divisible?
Answer :
Since the number is divisible by 12, it will also be divisible by its factors i.e., 1, 2, 3, 4, 6, 12. Clearly, 1, 2, 3, 4, and 6 are numbers other than 12 by which this number is also divisible.
Playing with numbers Exercise 3.5
Q. 1:Which of the following statements are true?
(a) If a number is divisible by 3, it must be divisible by 9.
(b) If a number is divisible by 9, it must be divisible by 3.
(c) A number is divisible by 18, if it is divisible by both 3 and 6.
(d) If a number is divisible by 9 and 10 both, then it must be divisible by 90.
(e) If two numbers are co-primes, at least one of them must be prime.
(f) All numbers which are divisible by 4 must also be divisible by 8.
(g) All numbers which are divisible by 8 must also be divisible by 4.
(h) If a number exactly divides two numbers separately, it must exactly divide their sum.
(i) If a number exactly divides the sum of two numbers, it must exactly divide the two numbers separately.
Answer :
(a) False
6 is divisible by 3, but not by 9.
(b) True, as 9 = 3 × 3
Therefore, if a number is divisible by 9, then it will also be divisible by
3.(c) False
30 is divisible by 3 and 6 both, but it is not divisible by 18.
(d) True, as 9 × 10 = 90
Therefore, if a number is divisible by 9 and 10 both, then it will also be divisible by 90.
(e) False
15 and 32 are co-primes and also composite.
(f) False
12 is divisible by 4, but not by 8.
(g) True, as 8 = 2 × 4
Therefore, if a number is divisible by 8, then it will also be divisible by 2 and 4.
(h) True
2 divides 4 and 8 as well as 12. (4 + 8 = 12)
(i) False
2 divides 12, but does not divide 7 and 5.
Q.2: Here are two different factor trees for 60. Write the missing numbers.
(a)
(b)
Answer :
(a) As 6 = 2 × 3 and 10 = 5 × 2
(b) As 60 = 30 × 2, 30 = 10 × 3, and 10 = 5 × 2
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