Question: 25 is 15 of what?
Answer: 25 is 15 percent of what number? (15% of what number is 25?) 25 is 15 percent of what number? If we substitue "what number" in the question "25 is 15 percent of what number?" with 166.67, we get this sentence: 25 is 15 percent of 166.67. Since this statement is true, we know that our answer is correct.