Question 95055
h=(-g/2)t^2+vt+s ... where g is gravitational acceleration (9.8 in meters), v is initial velocity, and s is starting height


80=-4.9t^2+20t+100 ... 0=-4.9t^2+20t+20 ... using quadratic equation gives


{{{t=(-20 +- sqrt(20^2-4*(-4.9)*20))/(2(-4.9))}}} ... t=-0.83 and t=4.91 ... the negative value is not realistic