Answer
$\displaystyle10+10\sqrt{2}$
Work Step by Step
We multiply the numerator and denominator by $\sqrt{2}+1$ and use the fact that $(a+b)(a-b)=a^2-b^2$ to simplify:
$\displaystyle \frac{10}{\sqrt{2}-1}=\frac{10}{\sqrt{2}-1}\cdot\frac{\sqrt{2}+1}{\sqrt{2}+1}=\frac{10+10\sqrt{2}}{2-1}=10+10\sqrt{2}$
