A ball is thrown straight up from the ground

Question

with an initial velocity of 30.4 m/s; at the
same instant, a ball is dropped from the roof
of a building 21.1 m high.
After how long will the balls be at the
same height? The acceleration of gravity is
10 m/s2.
Answer in units of s

Answer 1

d1 + d2 = 21.1 m.
(Vo*t - 0.5g*t^2) + (0.5g*t^2) = 21.1,
The 2nd and 3rd terms cancel:
Vo*t = 21.1,
t = 21.1 / Vo = 21.1 / 30.4 = 0.694 s.