20/(4-(10-8))
First do the operation in the furthest parentheses.
$$\begin{array}{l} \frac{20}{(4-\bf{(10-8)})}\\\\ \frac{20}{(4-(2))}\\\\ \frac{20}{(4-2)}\\\\ \frac{20}{2}\\\\ \frac{10}{1}\\\\ 10\\\\ \end{array}$$