Question

Two trains of equal lengths cross a pole in 11 sec and 15 sec. In how much time will they cross each other while running in the same direction? 
1. 83.5 sec
2. 86 sec
3. 82.5 sec
4. 88 sec

Answer 1

Correct Answer - Option 3 : 82.5 sec

GIVEN:

Time taken to cross a pole = 11 sec, 15 sec

CONCEPT:

When two objects are moving in the same direction the relative speed is the difference of their individual speeds.

Distance traveled in the crossing of two trains is the sum of lengths of the two trains.

FORMULA USED:

Speed = distance/Time

CALCULATION:

Let the length of each train be 165x m (L.C.M of 11 and 15)

Speed of faster train = 165x/11 = 15x m/s

Speed of slower train = 165x/15 = 11x m/s

Relative speed = (15x- 11x) = 4x m/s

Distance in crossing = 165x + 165x = 330x

T = 330x/4x = 82.5 sec

∴ In 82.5 sec they will cross each other while running in the same direction.