Force acts for 20s on a body of mass 20kg , starting from rest,...

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

$50 = V \times 10$

$⇒ V = 5m{s}^{- 1}$

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

$V = 0 + a \times 20$

$⇒ 5 = a \times 20$

$⇒ a = \dfrac{1}{4}m{s}^{- 2}$

Hence, required force will be

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