Question

A ball is thrown vertically upward from the ground with a velocity of 30 m/s. When is the displacement of the ball zero?

Answer 1

The displacement of the ball would be zero, when it will be at the ground:

$$t=2*\frac{v_0}{g}=2*\frac{30\:\rm m/s}{9.8\:\rm m/s^2}=6.1\:\rm s$$