yes, precisely. They needed an example for multiplying the opposit of a sum. the way you did was multyplying sums. Nothing wrong with it at all, but not what they wanted to explain.
Actually the 100-4 makes sense. You can multiply by 100 by simply adding two 0es or shifting the decimal point (or in Europe the decimal comma) two digits to the right. This is a mechanical step that doesn't require much thinking and goes quickly.
Then they substract the rest.
Imagine their example as:
there are 25 boxes with 100 cupcakes each. Someone steals 4 cupcakes from every box. How many are left?
See? Now it makes perfect sense
(and a belly ache for the thief LOL)