Question 42743
Use General Equation:
{{{f(x) = -16t^2 + vt + st^0}}} where {{{v}}} is the initial velocity and {{{s}}} is the height you started at
{{{f(x) = -16t^2 + 20t + 100}}}
When will the ball reach a height of 80 m?
{{{80 = -16t^2 + 20t + 100}}}
{{{0 = -16t^2 + 20t + 20}}}
*[invoke quadratic "t", -16, 20, 20]
About 1.9 seconds ....