One train is 100 m longer than another. When they move in opposite directions, they cross each other in 20 seconds. When they move in the same direction, the faster train crosses the slower train in 60 seconds. If the speed of the slower train is 36 km/hr, what is the speed of the faster train?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 72 km/hr
In both cases the trains cover the same distance, the sum of their lengths. Only the relative speed changes: the sum of the speeds in opposite directions and the difference in the same direction. So the two products of relative speed and time are equal, and the 100 m is not needed.
Let the faster train run at \(v\) km/hr
\((v+36)\times20=(v-36)\times60\)
\(v+36=3v-108\), so \(2v=144\)
\(v=72\) km/hr