a
(a)
Acceleration of system of blocks $=\frac{F}{8+2}=\frac{F}{10} \,ms ^{-2}$
Now, from free body diagram below, we see that
Reaction of $X$ and $Y$ is
$R=m_Y a=2 \times \frac{F}{10} \Rightarrow R=\frac{F}{5}$
Block $Y$ does not slips, if
Force of friction $=$ Weight of $Y$
$\Rightarrow \quad \mu R=m_Y g \Rightarrow 0.5 \times \frac{F}{5}=2 \times 10$
So, $\quad F=200 \,N$
