Question

Two trains of the same length but with different speeds pass a static pole in 4 sec and 5 sec respectively. in what time will they cross each other .when they are moving in the same direction?

Answer 1

let the speeds of the trains be A and B and
their length be L
A = L/4
B = L/5
now when they move in same direction
to cross one train, the other train has to cover 2L distance
now
relative velocity = L/4 - L/5 = L/20
therefore time taken = 2L/L/20 = 40 sec