Math Challenge (Solve problem; use answer in next question)

First, solve previous problem (to best of ability...don't worry if you tried and find out later it's incorrect...this is for fun! No correcting please).
Second, type a basic, algebra, geometry, or "symbol" math problem for next person to solve.
Third: The problem you ask is to include your response to the previous question anywhere in the problem.

The starter: What is the square root of 16?

First Response: Ans: 4 Solve: What does 4! = ____