Question

a ball thrown horizontally with velocity v from the top of a tower of height h reaches the ground in t second.If another ball of double the mass is thrown horizontally with velocity 3v from the top of another tower of height 4h it reaches the ground in(seconds)

Answer 1

the first thing to notice is that the horizontal velocity has no effect on the time of flight and that only vertical velocity and accelerations are to be considered to know the time of flight.

so initial vertical velocity = 0.acceleration = g.
(also mass is irrelevant unless there is a force calculation involved)
dist for first case = h,     4h -> for second case
So the only fact that needs to be considered here is height as gravity is constant.
S = 1/2  x (a   x  t square)  as u =0
in first case we get h = half a x (t square)
so let t1 be the time in second case =>    4h = 1/2 x(t1 square)
=> t1 square = 4 (t square)
=> t1 = 2t
so the answer is 2t.

Answer 2

T=√2h/g

= √2×4h/g

=2×√2h/g

= 2t