John bounces a ball off the ground, and it leaves the ground with a velocity of 10 m/s. how long will it take the ball to reach maximum height?

Explanation:

This is true because at maximum height, the velocity is 0

Gravity makes an upward-moving object move 9.8 m/s slower every second.

So if it leaves the ground at 10 m/s, it'll run out of upward speed in

(10 m/s) / (9.8 m/s²)  =  1.02 seconds

At that time, since it has no more upward speed, it'll start falling. So at that instant, it was exactly at its maximum height.