Which number pair is the odd one out: 4 : 16, 6 : 36, 7 : 56, 9 : 81?

Explanation

1. 01

   In 4 : 16, 6 : 36 and 9 : 81 the second number is the square of the first.

2. 02

   $7^2 = 49$, not 56 $(56 = 7 \times 8)$. So 7 : 56 is the odd one out.