A rock is thrown horizontally from a cliff with a speed of 15 m/s. It falls half the height of the cliff in the last three seconds of its fall. What is the total fall time? (use g = 9.81 m/s^2)

This is my attempt, since the rock is thrown horizontally it makes no difference to the fall time.

so:

$\displaystyle \frac{3}{\left(1-\sqrt1-0.5\right)}=10.24264\:secs$

Can you please confirm?