10. Solve: $(20-r)^{\frac{1}{2}}=r$

Step 1 ：Square both sides of the equation: $(20-r)^2 = r^2$

Step 2 ：Rearrange the equation to form a quadratic equation: $r^2 + r - 20 = 0$

Step 3 ：Solve the quadratic equation for $r$: $r = -5$ and $r = 4$

Step 4 ：\(\boxed{r = -5, r = 4}\)