(b) Let particle thrown with velocity $u$ and its maximum height is $H$ then $H = \frac{u^{2}}{2g}$
When particle is at height $H/2$, then its speed is $10\ m/s$
From equation $v^{2} = u^{2} - 2gh$
$(10)^{2} = u^{2} - 2g\left( \frac{H}{2} \right) = u^{2} - 2g\frac{u^{2}}{4g}\ \Rightarrow u^{2} = 200$
Maximum height $\Rightarrow H = \frac{u^{2}}{2g} = \frac{200}{2 \times 10} = 10\ m$