Question 494920
a rock falls from a tower that is 320 feet high.  As it is falling, its height is given by the formula h=320-16t^2.  How many seconds (in tenths) will it take to get to the ground.(h=0)?

My solution
0=-16t^2+320t
0=-4t(4t-80)
4t=0 or 4t-80=0
t=0 or t=20
0 doesn't answer the question so I get 20 seconds.  The answer key on my study sheet says 4.5 seconds.
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0=-16t^2+320  Not 320 t
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16t^2 = 320
t^2 = 20
t = sqrt(20) =~ 4.472 seconds