Why does GregMat/PrepSwift use this formula for finding the number of multiples in an interval?
Versus using this formula if it is more broadly applicable?
Your formula is arguably less broadly applicable. It doesn’t work as intended for decimal endpoints, whereas the other one works for all valid closed intervals. You can imagine extending your proposed formula to cover decimal endpoints, but it would require using ceils and floors in tandem.
Also, the second formula would feel like a random rule tailored specifically for this problem. The pedagogical issue is that if someone forgets it, there isn’t really an optimal way to re-derive it within the context of the GRE. Also, ideally you want to limit the number of formulas you memorize, so adding this one is kind of unnecessary.
The current formula sidesteps this issue because it relies on the concept of an arithmetic sequence (which is tested in some capacity). You can naturally derive this formula, rather than memorizing a “random” formula.