A stone is dropped from the top of a 20m high tower. 1sec later another stone is thrown downwards from the top. Both stones reach the ground simultaneously. If g is 10m/s2, the initial velocity of the second stone is

15 m/s

16 m/s

20 m/s

None

I dropped ⇒ initial velocity =0

⇒ time will be given by h=12gt^2

t=√2hg=√2×20 m10 m/s^2=2 sec

II both reaches simultaneously be so ti,e taken by IInd stone will be

2 sec−1 sec=1 sec

or t′=1 sec

putting it in h=ut'+12gt^2

& h=20 m,g=10m/s^2

we get

20=u×1+5×1

⇒u=15 m/s