Correct Answer - `270 m`

Time interval between beats (initially) `= (60)/(40) = 1.5` sec

Since `t = (2S)/(v)`

`1.5 = (2S)/(2v)`

where `S` is the initial distance and `v` is the speed of sound . As the man approches `90m` towards the mountain

`(2(S - 90))/(v) = 1` since drumming rate is `60//min`

Solving for `S` from the two relations we get

`(2S)/(2(S - 90)) = 1.5 , (S - 90) /(S) = (2)/(3)`

`rArr 3S - 3 xx 90 = 2S , S = 270 m`