We have the expression for the height H of the rock in function of the time r, when it falls from a height of 20 meters:

When r = 1 second, we have:

When r = 1.1 seconds, we have:

When r = 1.2 seconds, we have:

To know the time for each height, we have to work with the equation like this:

Then, we can calculate at which time the height is 15 meters:

When the height is 10 meters, r is:

When the height is 5 meters, r is:

The rock hits the ground when H=0. This happens when r is:
