Question: 40 is 5 of what?
Answer: 40 is 5 percent of what number? (5% of what number is 40?) 40 is 5 percent of what number? If we substitue "what number" in the question "40 is 5 percent of what number?" with 800, we get this sentence: 40 is 5 percent of 800. Since this statement is true, we know that our answer is correct.