Vfinal is 20m/s because speed coming down is the same as going up, and v initial is 0 because we can assume that this problem is the same as a drop problem.
so 400 = 2(9.81)x
solve for x and it is about 21.
so now we have the maximum height reached.
use x=0.5at^2 to solve for time to fall.
21=0.5(9.81)t^2
4.3=t^2
time=2.17 seconds aproximately (sorry i had no calculator you may want to check the math but the formulas are right.)Source(
