Page 53 - Start Up Mathematics_5

EXERCISE 3.1

1. Check the divisibility of the following numbers by 3 and 9.
(a) 723 (b) 9,604 (c) 8,325 (d) 2,01,364

2. Check the divisibility of the following numbers by 5 and 10.

(a) 2,685 (b) 9,99,900 (c) 36,412 (d) 25,540
3. Check the divisibility of the following numbers by 11.
(a) 75,691 (b) 2,37,405 (c) 32,95,71,836 (d) 7,986

4. Check the divisibility of the following numbers by 2, 3, 4, 5, 6, 8, 10 and 11. Colour
the box green if divisible and red if not divisible.

Number 2 3 4 5 6 8 10 11

5,381

81,620

61,800

65,175

4,26,972

2,98,53,300

Co-prime numbers

Two numbers are said to be co-prime if the only common factor between them is 1.
Example 10: Show that 21 and 32 are co-prime numbers.

Solution: Write the factors of 21 and 32.

21 32

3 7 4 8 A Challenge!
Write 3 pairs of numbers
2 2 2 4 which are co-prime.

2 2

21 = 3 × 7 32 = 2 × 2 × 2 × 2 × 2
We see that there is no common factor except 1 between 21 and 32. Thus 21
and 32 are co-prime numbers.

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