Use the concept of conservation of energy in that case the sum of initial kinetic energy and potential energy should be equal to the sum of final kinetic energy and potential energy, 

$$\begin{gathered} \Rightarrow mgh+\frac{1}{2}m{v}^2=mgh\text{'}+\frac{1}{2}mv{\text{'}}^2 \\ \Rightarrow v{\text{'}}^2=2gh+v^2-2gh\text{'} \end{gathered}$$

here, $h\text{'}=60 \mathrm{m}$, $h=100 \mathrm{m}$ and $v=10 \mathrm{m} {\mathrm{s}}^{-1}$, 

$$\begin{gathered} \Rightarrow v{\text{'}}^2=2\times 10\times 100+{10}^2-2\times 10\times 60 \\ \Rightarrow v\text{'}=30 \mathrm{m} {\mathrm{s}}^{-1} \end{gathered}$$