Question

A football is kicked into the air from an initial height of 4 feet. The height, in feet, of the football above the ground, is given by s(t) = -16t^2 + 50t + 4, where the t is time, in seconds, and t>= 0. At what time will the football be 25 feet above the ground?

A. 3.625 seconds
B. 0.5 seconds
C. 0.5 seconds or 3.625 seconds
D. 0.5 seconds or 2.625 seconds

According to the problem, which of these is correct?

Answer 1

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Answer 2

You want -16t^2 + 50t + 4 = 25
16t^2 - 50t + 21 = 0

It factors:
(2t - 1)(8t - 21) = 0
t = 1/2 or t = 21/8

The ball will be 25 ft above the ground after 1/2 second and then on its way down again
at a time of 21/8 seconds. (21/8 = 2.625)