This is a problem from Test of Mathematics, TOMATO Objective 8, useful for ISI and CMI Entrance. Try out this problem. (by Rahul Roy)
Two trains of equal length L,travelling at speeds v1 and v2 miles per hour in opposite directions,take t seconds to cross each other .then L in feet is:
Total distance required to cover by one train is 2L.
By considering one train at rest then speed of other train is v1+v2.
converting into feet we get
L = 11(v1+v2)T 15