Two trains, 200 m and 250 m long, are moving in the same direction at speeds of 80 km/hr and 60 km/hr, respectively. How long (in seconds) will it take for the faster train to completely pass the slower one?

Correct option is A

Given:

Length of Train A (faster) = 200 m

Length of Train B (slower) = 250 m

Speed of Train A = 80 km/h

Speed of Train B = 60 km/h

Both moving in same direction

Concept Used:

When two objects move in the same direction, relative speed is the difference of their speeds.

Time to cross = $$\frac{\text{Total Distance}}{\text{Relative Speed}}$$​ 

Solution:

Relative Speed = 80 - 60 km/h = 20 km/h = $$\frac{20 \times 1000}{3600} = \frac{50}{9} \text{ m/s}$$​​

Total Distance = 200 + 250 = 450 m

Time = $$\frac{450}{50/9} = 450 \times \frac{9}{50}$$​ = 81 seconds