(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.