Please answer this question quickly

Dear Student,

Work done by gravitational force 

 $$\begin{gathered} =mgh (here h = 20 cm as displacement along gravitational force is = radius of hemisphere) \\ = 2 \times {10}^{-3} \times 9.8 \times 20 \times {10}^{-2} =39.2 Joules \end{gathered}$$
  = 3.92 mJ
Regards