Let total height =H
Total Time of ascent =T
So, H=uT−21gT2
Distance covered by ball in time (T−t)sec is y=u(T−t)−21g(T−t)2
So, distance covered by ball in last tsec h=H−y=[uT−21gT2]−[u(T−t)−21g(T−t)2]
By solving and putting T=gu
we will get, h=21gt2