A ball is thrown with an initial velocity of $20m{s}^{-1}$. Find the time of flight by the body. (Take $g=10m{s}^{-2}$)

The correct option is A $4s$
Let $a$ be the acceleration of the ball, $u$ be the initial velocity, $v$ be the final velocity and $t$ be the time taken.

Initially the ball was moving in the upward direction. At maximum height, velocity will be zero, (v = 0). At that point, the ball will change the direction of motion and fall downwards.
Since, acceleration due to gravity is opposite to motion, $a=-10m{s}^{-2}$
From first equation of motion, $v=u+a$
$\Rightarrow$$0=20+(-10)t$
$\Rightarrow$$t=2s$
Thus, the ball takes $2s$ to reach the top. Similarly to reach the ground from the top point is also $2s$. Thus total time or time of flight is $4s$.