Force acts for 20   s on a body of mass 20   kg , starting from rest, after which the force ceases…

For the second case when body is moving with constant velocity, we can write

$50 = V \times 10$

$\Rightarrow V=5 \mathrm{m} {\mathrm{s}}^{-1}$

Now this velocity if the final velocity for the first case, therefore

$V = 0+a\times 20$

$\Rightarrow 5=a\times 20$

$\Rightarrow a=\frac{1}{4}\mathrm{m} {\mathrm{s}}^{-2}$

Hence, required force will be

$F=ma=20\times \frac{1}{4}= 5 \mathrm{N}$