Question 168270
By the moment that the object hits the ground, h=0.
Thus, substituting,

0=80t-16t<sup>2</sup>
factor out t

t(80-16t)=0

t=0    or   80-16t=0
            16t=80
              t=5

SO, it's 0 or 5 seconds.
HOwever, at t=0, the object is not falling but was about to go up with the initial velocity of 80 ft/sec

So, the answer is that t=5.


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