Two bodies begin to fall freely from the same height but the second falls T second after the first. The time (after which the first body begins to fall) when the distance between the bodies equals L is:

Explanation

Let the time of fall of the 1st body be t seconds.
Time of fall of second body = `t-tau` .
Distances of free fall of the bodies in the above time intervals respectively are
`H_1=(g t^2)/2 , H_2=(g(t-tau)^2)/2`
Therefore `l=H_1-H_2= g t tau -1/2 g tau^2`
`therefore t=l/(g tau)+tau/2`