Question 487026
when the height reaches to 64 ft, 
-16t^2 + 80t = 64
-16t^2 + 80t - 64 = 0
t^2 - 5t + 4 = 0
(t - 4) (t - 1) = 0
t = 4 or t = 1
thus the shortest time is t = 1 second
John10:)