Aptitude - Odd Man Out and Series - Discussion

2 is also a prime number. So 31 cannot be the answer even if we consider this question in terms of prime numbers.

This question is solved on the basis of perfect squares. Subtract each adjacent number and you will get a perfect square, except:

91-56 = 35.

35 is not a perfect square, so we make it 36 by adding 1 to 91. Thus the final series will be as follows:

1, 2, 6, 15, 31, 56, 92
where subtracting any two adjacent numbers will give a perfect square.

I hope this helps.

2 - 1 = 1(odd),
6 - 2 = 4(even),
15 - 6 = 9(odd),
31 - 15 = 16(even),
56 - 31= 25(odd),
But 91 - 56 = 45 (odd) so that 91 is the wrong number here.

Easy to Understand. Thanks everyone for explaining the answer.

Actual answer by easy method.

1 is odd then two are even 2, 6 then two are odd 15, 31 then two are even 56, 91 but here 91 is odd so answer. Is 91 which we same get with previous method.

According to me 31 should be the answer because its the only prime no in the question?

Answer should be 31.

2, 5, 15, 131, 530, 13257 find out wrong number of the series.

31 is the only prime number.

Why not the answer is 31? Some one explain.

If we see prime numbers. Then 31 will be the answer. So which is the correct answer?

Why not 31? It's the only prime number.