Given a number x and a non-negative integer n, compute xn. For example, 210=1024 and 34=81. You could multiply x by itself n times. That takes n multiplications.
But there is a recursive approach that uses far fewer operations. Can you see it? Think about what x10 and x5 have in common. Read the problem statement carefully, noting the constraints. Try to identify the recursive structure before looking at the solution.