Chapter 6 - Section 6.7 - Quadratic Equations and Problem Solving - Exercise Set - Page 466: 13

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$h=-16t^2+64t+80$ $0=-16t^2+64t+80$ $0/-16=(-16t^2+64t+80)/-16$ $0 = t^2-4t-5$ $t^2-4t-5$ $t^2-5t+t-5$ $t(t-5)+1(t-5)$ $(t-5)(t+1)$ $t-5=0$ $t=5$ $t+1=0$ $t=-1$ (this answer is not valid since we cannot have a negative time)