> I’m looking for them to find the right abstraction—one that will make it easy to extend the problem and for junior devs to look at the solution function and understand exactly what it’s doing. Almost no one passes.

If no one passes, it means you are instead asking them to recreate what YOU would do, and basically asking them to be a mind reader. A good interviewer would know that there is more than 1 solution to everything, and that there are equal, but completely different solutions to the same problem. Someone could abstract the problem completely different from yours, but still be correct.