Page 61 - Math Skill - 4

Factors and Multiples 59

Exercise 4.3

1. Write all the odd numbers between 1 and 20.

2. Write all the prime numbers between 20 and 40.
3. Write all the composite numbers between 20 and 50.

4. Which is the only even prime number?
5. Encircle the even numbers and tick the odd numbers.

(a) 13, 24, 62, 90, 43 (b) 53, 104, 237, 648, 913
6. Encircle the prime numbers.
(a) 3, 14, 17, 16, 20 (b) 23, 30, 41, 63, 80 (c) 29, 92, 67, 11, 200

7. Encircle the composite numbers.
(a) 6, 16, 23, 51, 37 (b) 3, 13, 33, 23, 53 (c) 4, 5, 17, 27, 39

8. Do as directed.
(a) Write all the prime numbers below 35.

(b) Write all the composite numbers below 25.
(c) Write the number which is neither prime nor composite.

9. Write the prime factors of the following numbers by drawing factor trees.
(a) 24 (b) 36 (c) 72 (d) 45 (e) 90

Test of Divisibility

A number is said to be divisible by another number if on dividing, no remainder is left. To test,
if a number is divisible by a certain number without doing actual division, we can follow the
following rules.

Divisibility By 2

A number is divisible by 2, if the digit at the ones place is 0, 2, 4, 6, 8.
i.e. 16, 28, 194, 75280 all are divisible by 2 whereas 13, 267, 1845, 1009 are not divisible by 2.

In other words, all even numbers are divisible by two.

Divisibility By 3

A number is divisible by 3, if sum of all the digits is divisible by 3.
For example,

(a) 4320 (b) 175
Sum of digits = 4 + 3 + 2 + 0 = 9 Sum of digits = 1 + 7 + 5 = 13

Sum of digits is a multiple of 3. Sum of digits is not a multiple of 3.
Thus, number 4320 is divisible by 3. Thus, number 175 is not divisible by 3.