A train travelled at one-thirds of its usual speed, and hence reached the destination minutes after the scheduled time. On its return journey, the train initially travelled at its usual speed for minutes but then stopped for minutes for an emergency. The percentage by which the train must now increase its usual speed so as to reach the destination at the scheduled time, is nearest to
A train travelled at one-thirds of its usual speed, and hence reached the destination minutes after the scheduled time. On its return journey, the train initially travelled at its usual speed for minutes but then stopped for minutes for an emergency. The percentage by which the train must now increase its usual speed so as to reach the destination at the scheduled time, is nearest to
Solution
A train has two journeys to analyze:
Forward journey: Travels at usual speed, arrives 30 minutes late
Return journey: Travels normally for 5 minutes, stops 4 minutes, then needs to speed up
We need to find the percentage increase in speed required for the return journey.
Let's define our variables:
Usual speed = S
Usual time for the journey = T
Distance = D = S × T
Forward Journey Analysis:
Speed used = (one-third of usual speed)
Time taken =
When speed is reduced to , the time increases by a factor of 3 since Speed × Time = Distance (constant).
Finding T:
Delay = Time taken - Usual time = minutes
Therefore: minutes
The usual journey time is 15 minutes.
Initial Conditions:
Total scheduled time = minutes
Distance to cover =
Journey Breakdown:
First 5 minutes: Travels at usual speed S
Distance covered =
Remaining distance =
Time Analysis:
Stop time: 4 minutes
Remaining time = minutes
Speed Calculation:
Distance remaining =
Time remaining = 6 minutes
Required speed =
Speed Increase:
Usual speed =
New speed =
Increase =
Percentage Increase:
Percentage =
Percentage =
The train must increase its usual speed by 66.67% (approximately 67%) to reach the destination on time.
In time-speed-distance problems, when speed changes by a factor, time changes by the reciprocal factor (assuming distance stays constant).