Four balls are dropped from the top of a tower at intervals of one second. The first ball reaches the ground after four seconds of dropping. What are the distances between first and second, second and third, third and fourth balls at this instant?

This is the case of free fall.
Let H be the height of tower.
H=0×t+(1/2)gt^2,
H=(1/2)×10×t^2=5t^2
First ball reaches ground in 4s
H=5(4)^2=80m
distance between ball 1st and 2nd,
d1=(1/2)gt^2−(1/2)g(t−1)^2=(10/2)(t^2−(t−1)^2)
d1=5(4^2−3^2)=35m
distance between ball 2nd and 3rd,
d2=(10/2)(3^2−2^2)=25m
distance between ball 3rd and 4th,
d3=(10/2)(2^2−1^2)=25m=15m