I fire a tennis ball from an air cannon, straight up from ground level, at initial speed v0. At

Question

the instant it reaches the top of its trajectory, I fire a second tennis ball at the same initial
speed. Air resistance is negligible. At what fraction of the maximum height reached by the
first ball do the two collide?

Answer 1

start ball one at max height:
hf=maxheight-1/2 g t^2
ball two:
hf=vo*t-4.9t^2

but t is the same, and hf is the same:

set the two equations equal, and solve for t.
then having t, find hf.