Two trains, each of length 200 m are running on parallel tracks. One overtakes the other is 20 s and one crosses the other in 10 s.
The velocities of the two trains are

Options

A. `5 ms^(-1), 10ms^(-1)`
B. `10 ms^(-1), 30ms^(-1)`
C. `15 ms^(-1), 20ms^(-1)`
D. `20 ms^(-1), 21ms^(-1)`

Answer

B

Explanation

Let velocities of two trains be u and v.
Relative velocity while overtaking
` = u -v`
Relative velocity while crossing
` = u + v`

Distance travelled in each case
= 200 + 200 = 400 m
`rArr 20 = (400)/(u-v)`
` therefore u -v = (400)/(20) = 20" "..(i)` ,brgt `u = v = (400)/(10) = 40 " "...(ii)`
Adding Eqs. (i) and (ii), we get
`u = 30ms^(-1), v = 10 ms^(-1)`