A rock falls from a cliff that is 320 m high how long will it take the rock to reach the ground

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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. -------------- 0=-16t^2+320 Not 320 t ----------- 16t^2 = 320 t^2 = 20 t = sqrt(20) =~ 4.472 seconds

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