Make a factor tree for 16

         16
          ∧
     4   X  4
      ∧      ∧
   2 X 2   2 X 2

The answer is 2 x 2 x 2 x 2 or 2⁴   This would have worked as well had you broken 

down 16 into 8 x 2.  You would get the same answer as the 8 would break down into 4 x 2 and the 4 would break down into 2 x 2.