\text{Since we have the height in feet, use equation} & & y & = -16t^2 + y_0\\\text{The initial height} \ y_0 = 120 \ feet. & & y & = -16t^2 + 120\\\text{The height when the ball hits the ground is} \ y = 0, \ \text{so} & & 0 & = -16t^2 + 120\\\text{Solve for} \ t & & 16t^2 & = 120\\& & t^2 & =  \frac{120} {16} = 7.5\\& & t & = 2.74 \ \text{or} \ t = -2.74 \ seconds.
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