Aptitude - Odd Man Out and Series - Discussion

81 is divisible by 9, so it isn't a prime number. 9x9=81. the rest of the numbers can not be divided into evenly without a remainder. For example 5 is a prime because if you divide it by 2 you will have a remainder. 5 is only divisible by 5 or by 1.

also stated above was the formula: tn= t(n-1)+2n
where n= 1,2,3,4,...etc.
and t= 41,43,47,53,...etc.

If you plug in the numbers it doesn't equal the 2nd term which is 43.
let t= 1st term 41
n= 1

Example with above formula:tn= 41(1-1)+2(1)
41(0)+2 = 2?
I do understand t+2n
41+2(1)= 43
43+2(2)= 47
47+2(3)= 53

I am not trying to be picky, I was really trying to understand.
thanks for the above solutions!

The difference order is in2, 4, 6, 8, 10, 12, and here it fails with 81.

Read the answer properly, it say that except 81 all are prime number. To all genius.

81 is the only one which is divisible by 3.

Why not 73?

BUT why not 73 because its also not fit in series with 81? both are not fit isn't it?

It's 81.

That's because NO OTHER NUMBER IS PERFECTLY DIVISIBLE BY 3, EXCEPT 81.

What is a prime no?

@Murali.

How did you wrote the series of prime no. Can you explain it?

t = 41 + 2 * 1 = 43
t = 43 + 2 * 2 = 47
t = 47 + 2 * 3 = 53
t = 53 + 2 * 4 = 61
t = 61 + 2 * 5 = 71
t = 71 + 2 * 6 = 83.

In the given Series there 73 followed by 71, so the answer must be 73.

Why its ans can't be 47?

41 4>1
43 4>3
47 4<7
53 5>3
61 6>1
71 7>1
73 7>3
81 8>1

Every 10th digit no is more than except 47, so the answer is 47. Why this answer is wrong?