Question

Let A = {x l 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. None of these

Answer 1

Answer: (C)

91

Answer 2

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 = ¹⁰P₂ +1 (including 1)