When is the difference of
two decimals an integer

For the difference between two decimals to be an integer (whole number), the difference between the decimal part of the two numbers should be zero.
In this case, we would have a zero after the decimal point, which can simply be ignored.

Examples:
2.5 - 1.5 = 1.0 which is 1
6.729 - 2.729 = 4.000 which is 4
9.12 - 7.12 = 2.00 which is 2

Hope this helps :)