Question

A smart-phone is thrown upwards from the top of a 240-foot building with an initial velocity of 32 feet per second. The height h of the smart-phone after t seconds is given by the quadratic equation h=-16t^2+32t+240. When will the smart-phone hit the ground?

Answer 1

when it hits the ground, h = 0

so solve:
16t^2 - 32t - 240 = 0
t^2 - 2t - 15 = 0
(t-5)(t+3) = 0
t = 5 or t = -3, which is not acceptable

it will hit the ground after 5 seconds

Answer 2

At what time will the smartphone reach its maximum height?