physcis

s=1/2at^2
s=45m
a=10 m/s^2
Rearange to solve for t
t=sqrt(2s/a)=sqrt(90m/10 m/s^2)=sqrt(9 s^2)=3 s
the stone falls for 3 seconds

at the end of one second s=1/2 10 m/s^2 (1 s)^2=5m
at the end of two seconds s=1/2 10 m/s^s (2 s)^2=20 m
at the end of three seconds , that's a given 45m

Rap