Two trains `A` and `B` of length `100m` and `200m`, respectively, are approaching each other on parallel tracks. If they take `15 s` to pass each other and velocity of `A` is three that of `B`, find their velocities.

Options

A. 5,15
B. 30,10
C. 12,4
D. 15,5

Answer

D

Explanation

`v_(A//B)=3v+v=4v`, now train `B` is at rest and train `A` is moving with velocity `4v`.

Time which trains cross each other
`t=(l_(A)+l_(B))/(v_(A//B))=(100+200)/(4V)=300/(4v)`
`15=300/(4v) implies v=5m//s`
`v_(A)=3v=15 m//s`
`v_(B)=v=5m//s`