**(11) Write five pairs of prime numbers less than 20 whose sum is divisible by 5.**

Sol: **Prime numbers less than 20 are 2, 3, 5, 7, 11, 13, 17 and 19**

- 2 + 3 = 5 (divisible by 5)
- 2 + 13 = 15 (divisible by 5)
- 3 + 7 = 10 (divisible by 5)
- 3 + 17 = 20 (divisible by 5)
- 7 + 13 = 20 (divisible by 5)

**(12) Fill in the blanks**

(a) A number which has only two factors is called a **prime number.**

(b) A number which has more than two factors is called a **composite number.**

(c) 1 is neither **prime** nor **composite.**

(d)The smallest prime number is **2.**

(e)The smallest composite number is **4.**

(f)The smallest even number is **2.**