IBPS SO Prelims Exam Model Paper 10

The number on the right of the colon is 1 fewer than half of the number on the left of the colon as shown below:
42 ÷ 2 = 21 → 21 - 1 = 20
Similarly,
64 ÷ 2 = 32 → 32 - 1 = 31
Hence, 31 is related to 64 the same way 20 is related to 42.