Time and Distance
About 6 min · Quantitative Ability · English
Time worked out from the lesson's 391 words at 150 words a minute, plus a minute for each question.
Time and Distance is a cornerstone topic in the Quantitative Ability section of HSSC CET. It tests your ability to apply the fundamental relationship between speed, distance, and time to real-world scenarios involving trains, boats, and moving objects.
Key ideas
- Basic Relationship: Distance = Speed × Time. This is the foundation—rearrange it to find any unknown when two values are given.
- Unit Conversion: To convert km/hr to m/s, multiply by 5/18. To convert m/s to km/hr, multiply by 18/5.
- Relative Speed (Same Direction): When two objects move in the same direction, relative speed = difference of their speeds. The faster object "gains" on the slower one.
- Relative Speed (Opposite Direction): When two objects move towards each other, relative speed = sum of their speeds. They close the gap faster.
- Train Crossing a Stationary Object: When a train crosses a pole or a person, the distance covered equals the train's own length.
Watch out for
- Adding speeds instead of finding relative speed for same-direction problems → When objects move in the SAME direction, subtract speeds. Add when they move in OPPOSITE directions (towards or away from each other).
- Forgetting to add both lengths when trains cross each other → Both trains must completely pass each other. Total distance = L₁ + L₂, not just one train's length.
- Mixing units without conversion → If speed is in km/hr and length in metres, convert speed to m/s FIRST. Many errors happen due to inconsistent units.
Quick reference
- Distance = Speed × Time; rearrange for any unknown.
- km/hr to m/s: multiply by 5/18; m/s to km/hr: multiply by 18/5.
- Opposite directions → ADD speeds; Same direction → SUBTRACT speeds.
- Train crossing anything = (Train length + Object length) / Speed.
- Boat still water speed = (Downstream + Upstream) / 2.
Check yourself: 3 questions
1. A train 150 metres long passes a telegraph post in 10 seconds. What is the speed of the train in km/hr?
2. A boat can travel 30 km downstream in 2 hours and 20 km upstream in 4 hours. What is the speed of the boat in still water?
3. A car travels from Delhi to Gurugram at a speed of 60 km/hr and returns at a speed of 40 km/hr. What is the average speed for the entire journey?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 18 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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