when the numbers after the decimal are the same. like 4.3090-3.3090. in this case, all the numbers after the decimal are the same, so the answer is going to be a whole number
I'm adding on to the other person's answer (I know this is really late since this is four years ago, but I'm doing it for the points). The difference of two decimals can also be an integer when they both have different signs and the decimal parts add up to 1.
