Question

Let A = {x I x is a prime number and x < 30}. The number of different rational numbers whose numerator and denominator belong to A is

(A) 90

(B) 180

(C) 91

(D) of these

Answer 1

Correct option**(C) 91**

A = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29}.

A rational number is made by taking any two in any order. So, the required number of rational numbers is (including 1)

10P2 + 1 = 10!/8! + 1 = 90 + 1 = 91