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1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20

21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48 49 50
51 50 53 54 55 56 57 58 59 60

61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80

81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 100

So the numbers cross out have more than two factors. Such numbers are called

composite numbers .

Co-prime Numbers
Two numbers which have only '1' as their common factor are called co-prime

numbers.

Example 1 : Which two numbers are co-prime number :- 16, 25, or 10, 15
Solution : (a) Factors of 16 are : 1, 2, 4, 8, 16
Factors of 25 are : 1, 5, 25 30, 35
16 and 25 have only 1 as their common factor. Hence they co-prime numbers.
(b) Factors of 10 are : 1, 2, 5, 10
Factors of 15 are : 1, 3, 5, 15
10 and 15 have two common factors, 1 and 5. Hence they are not co-prime
number.

E xercise 3.1

1. Select prime and composite numbers :

34, 65, 61, 53, 49, 93, 86, 83, 37, 33

2. List all the prime numbers between :
a. 20 and 30 b. 45 and 65 c. 70 and 95

3. Write all the composite numbers between :
a. 15 and 35 b. 40 and 60 c. 65 and 85

4. Write the prime number just less than :

a. 67 b. 44 c. 32 d. 98

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Mathematics In Focus - 5