TNPSC Group IV · Bite-size lessons
Time and Distance
About 6 min · Aptitude and Mental Ability Test · English
Time worked out from the lesson's 407 words at 150 words a minute, plus a minute for each question.
Time and Distance is a core quantitative aptitude topic, but it is not listed by name in the official TNPSC Group IV Aptitude and Mental Ability syllabus, which lists Time and Work. This topic tests your understanding of the relationship between speed, distance, and time—concepts you encounter daily but must apply precisely under exam conditions.
Key ideas
- Fundamental Relationship: Speed = Distance ÷ Time. This single formula, rearranged appropriately, solves most problems. Distance = Speed × Time. Time = Distance ÷ Speed.
- Unit Conversion is Critical: Converting km/hr to m/s requires multiplying by 5/18. Converting m/s to km/hr requires multiplying by 18/5. Wrong units = wrong answer.
- Relative Speed (Same Direction): When two objects move in the same direction, relative speed = difference of their speeds. The faster object "catches up" at this rate.
- Relative Speed (Opposite Direction): When two objects move toward each other, relative speed = sum of their speeds. They approach each other faster.
- Train Crossing a Stationary Object: The train must cover its own length to completely cross a pole or a person (length negligible).
Watch out for
- Forgetting to add train length: When a train crosses a platform, students often use only the platform length. Correct fix: Always add train length + object length for platforms, bridges, and other trains.
- Using wrong relative speed formula: Students add speeds when objects move in the same direction. Correct fix: Same direction = subtract speeds; opposite direction = add speeds.
- Ignoring unit conversion: Mixing meters with km/hr leads to absurd answers. Correct fix: Convert everything to consistent units (usually m/s for train problems) before calculating.
Quick reference
- Speed = Distance/Time — rearrange as needed
- km/hr to m/s: × 5/18 | m/s to km/hr: × 18/5
- Train crosses pole: covers its own length only
- Train crosses platform: covers train length + platform length
- Opposite direction: add speeds | Same direction: subtract speeds
Check yourself: 3 questions
1. A train 120 metres long passes a pole in 6 seconds. What is the speed of the train in km/hr?
2. A man can row downstream at 15 km/hr and upstream at 9 km/hr. What is the speed of the man in still water?
3. Two trains are running in opposite directions with speeds of 45 km/hr and 55 km/hr respectively. If the length of each train is 100 metres, in how many seconds will they cross each other?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 17 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
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