Two trains take `3s` to pass another when going in the opposite directions but only `2.5s` if the speed of one is increased by `50%`. The time one would take to pass the other when going in the same direcrtioin at their original speed is

Options

A. 10 sec
B. 12 sec
C. 15 sec
D. 18 sec

Answer

C

Explanation

`t = ((l_(1) + l_(2))/(v_(1) + v_(2)))`
`t' = (l_(1) + l_(2))/((3)/(2) (v_(1)) + (v_(2)))`
`t'' = (l_(1) + l_(2))/((v_(1)) - v_(2))`