
(a) $Given,\ m_{1} = 1kg,\ m_{2} = 2\ kg\ and\ g = 10m/s^{2}$
$a = \left( \frac{m_{2} - m_{1}}{m_{1} + m_{2}} \right)g$
$= \left( \frac{2 - 1}{1 + 2} \right)10 = \frac{10}{3}$
$S = \frac{1}{2}a\ t^{2}$
$= \frac{1}{2} \times \frac{10}{3} \times 4 = \frac{20}{3}$
$m = \frac{2 \times \frac{20}{3} - 1 \times \frac{20}{3}}{3} = \frac{20}{9}$