Question

Which one of these numbers is not like the others 25,16,49,63,81

Answer 1

It's 16, the first number is odd and the second number is even, unlike the others.

That's what I think.

Answer 2

25,16,49,63,81

It's 63. Why?
We notice that each number is a perfect square at the exception of 63:

5² =25
4² =16
7² =49
9² =81, so the number 63 which is 7 x 9 is the odd man in this series