Question 58464
let t be the time when the rocket hits the ground.
0=1960t-4.9t^2
0=4.9t(400-t)
4.9t=0, t=0 (when the rocket hits the ground)
400-t=0
400=t
the rocket hits the ground after 400 seconds or 6 2/3 min.


formula used: h=rt-4.9t^2