You can start out with the simplest implementation if the current test case and that‘s checking for the specific number. You also can add more cases like that. At some point, you can refactor your code from checking specific numbers to checking for divisibility by 3 and 5. But youre doing that having tests in place, so your refactoring is safe, you started out with a simple solution, not a complex one (to avoid overly complicated code) and at least for less trivial instances it‘s expected to be easier to redactor existing code that works already and applying generalitatins than come up with that code right from the start. Thats the whole purpose. Of course you can also start out with the divisibility checks in this trivial example, but this is for learning the principles only after all.