Near the end of the episode when Cory is telling Feeny the "correct" answer to the word problem, his book is first on top of his binder, then under it, then back on top between shots.
At the end of the episode, Mr. Feeny reveals the "correct" answer to the math problem: there is no answer. This is actually quite incorrect. It would take Al and Fred ~3.43 minutes to wash the car together.