c
Time to move at upper extreme
$=\frac{T}{2}=\frac{\pi}{\omega}=\pi \sqrt{\frac{m}{k}}=\frac{\pi}{10} \mathrm{sec}$
Distance moved by lower block
$=\frac{1}{2} g t^{2}=\frac{1}{2} \times 10 \times \frac{\pi^{2}}{100}=50 \mathrm{cm}$
Seperation between the blocks $(20+10+50) \mathrm{cm}$ $=80 \mathrm{cm}$
