Answer:
V = a t
t1 = 15 / 9.8 = 1.53 sec time to reach 0 speed
t2 = t1 = 1.53 sec time to reach point thrown
t3 = 5 - 2 * 1.53 = 1.94 sec time to fall height of cliff
H = V0 t - 1/2 g t^2 time to fall height of cliff (if V0 positive)
V0 = -15 m/s since it will return to thrown height with speed thrown up
Signs chosen must be consistent
H = -15 * 1.94 - 1/2 9.8 * 1.94^2 = -47.5 m where H is height of cliff
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