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.

In this problem they are adding the result with square value of the natural numbers.
I mean.

1,
1(result) + (1*1) = 2.
2(result) + (2*2) = 6.
6(result) + (3*3) = 15.
15(result) + (4*4) = 31.
31(result) + (5*5) = 56.
56(result) + (6*6) = 92.

Now we are getting 92 not 91.. So 91 is the wrong number of the given.

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.

@Kumar.

Your concept is OK. But same time you took the wrong concept.

First minus 1 by 2.

Then minus 6 by 2 how is it correct? OK now you understand.

But your thing is good.

10 teams participating in national football league. If each team plays the other team two times, the total number of games the league will play is.

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.

1 + (1*1) = 2.
2 + (2*2) = 6.
6 + (3*3) = 15.
15 + (4*4) = 31.
31 + (5*5) = 56.

56 + (6*6) = 92 (91 is wrong number).

2-1,6-2,15-6,31-15,56-31,91-56
1,4,9,16,25,45
last number is clearly wrong because those are perfect squares

2-1 = 1*1
6-2 = 2*2
15-6 = 3*3
31-15 = 4*4
64-31 = 5*5

But 91-56 = 6*6 is not happening.

So 91 is odd no.

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