An integer is a whole number that can be positive, negative, or zero, while a whole number includes all the non-negative integers (0 and positive integers).
Among the options provided:
- 10 is both an integer and a whole number.
- -4 is an integer but NOT a whole number.
- 25 is both an integer and a whole number.
- 2 fifths (2/5) is not an integer.
- 3.5 is not an integer.
Therefore, the number that is an integer but NOT a whole number is -4.