The variables $\displaystyle u$ and $\displaystyle v$ take positive values such that $\displaystyle \frac{1}{u}+\frac{1}{v}=\frac{1}{20}$. If $\displaystyle s=u+v$, find the least value of $\displaystyle s$ as $\displaystyle u$ varies.

Since they want the least value of s, my guess is that differentiating s and equating that equation to zero will get me the answer. but i cant figure of anything to differentiate s with respect to