We don't need to know about the cliff.
If the ball was thrown horizontally at 15 m/s and hit the ground 45 meters away, then it had to be sailing for (45 m) / (15 m/s) = 3 seconds in order to cover that distance.
Answer: 3s
Explanation: initial horizontal velocity of 15m/s is independent of the acceleration due to gravity, therefore, a=0
S= it +1/2at2
45=15*t +1/2*0*t
45=15t
t=45/15
t=3s
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