Answer
$$L=\frac{r^2}{20}$$
Work Step by Step
Solving the equation by isolating the given variable using the rules of Algebra, we find:
$$ 2\sqrt{5L}=r\\ \left(\sqrt{5L}\right)^2=\left(\frac{r}{2}\right)^2\\ L=\frac{r^2}{20}$$
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