Is it correct to say "15 percent less than 25"?
To me, it doesn't make 100% sense.
What you mean is understandable, even if it's an unusual way to say it.
If you mean the 85% of 25, then it's correct to say "15% less than 25." From a grammatical point of view, the phrase is correct.
It is not intuitive. Rather, say 85% of 25 or the answer itself (21.25).
I finally figured out why the sentence sounds strange to me. What a relief.
As a theoretical computer scientist, a red little lamp lights up in my head as soon as I hear something that doesn't immediately type check. In this case the red lamp lights up for the following reason:
When hearing "... less than 25" I expect it to be preceded by something you can subtract from 25. "1 less than 25" and "half of 10 less than 25" for instance, sound perfectly fine to me as both "1" and "half of 10" represent a number. "15 percent" on the other hand does not.
I suppose that the reason it still makes sense is that there is an implicit "of 25" in the middle. If written explicitly, like
"15 percent of 25 less than 25"
the red light stays off in my head :)
(If "15 percent less than 25" it was written right after a sentence like "15 percent of 100 less than 25" I think more people would react though.)
I do understand that "right" and "wrong" is, when it comes to natural language, loosely defined, and boils down to what most people usually say and write. In this case, as pointed out by others here, "15 percent less than 25" is perfectly correct as it sounds fine to most people!