A tennis ball is thrown straight up with an initial speed of 20 m/s.  How long does it take to reach the top of the flight?

2.04 seconds.

Explanation:

We use an equation of motion of a body acting under gravity:

v = u + gt    where u = initial velocity, g = acceleration due to gravity (9.81 m /s^-2) , v = final velocity and t = the time in seconds.

At the top of the upward journey v = 0 so we have:

0 = 20 - 9.81t     (the  a is negative because the gravity acts downwards).

9.81t = 20

t = 20/9.81

= 2.04 seconds.