I am asked to find the geometric means of 8 and 20

I set up this formula:

8 x

_ __

X 20

edit: the formatting came out inaccurate, but the formula that I set up was 8 over X and X over 20.

But my answer came out to be a decimal answer. The back of my textbook has the answer 4 squareroot 10

Please help me! Also, please elaborate the steps with how to get that answer. Thank You!