RRB NTPC · Bite-size lessons

Time, Distance and Speed

About 6 min · Mathematics · English

Time worked out from the lesson's 422 words at 150 words a minute, plus a minute for each question.

This topic tests your ability to translate real-world motion scenarios into mathematical relationships and solve for unknowns efficiently.

Key ideas

• Fundamental relationship: Distance = Speed × Time. Any problem ultimately reduces to this equation or a variant. Speed = Distance/Time, Time = Distance/Speed.
• Unit conversions are non-negotiable: km/h to m/s multiply by 5/18; m/s to km/h multiply by 18/5. Many train problems give platform/train length in metres and speed in km/h.
• Relative speed — same direction: When two objects move in the same direction, relative speed = |Speed₁ - Speed₂|. Used when one train overtakes another or a man walks inside a moving train.
• Relative speed — opposite direction: When two objects move toward each other, relative speed = Speed₁ + Speed₂. Critical for head-on train collisions or crossing problems.
• Average speed ≠ arithmetic mean of speeds: For a journey with multiple legs at different speeds, Average Speed = Total Distance / Total Time. Never just average the speeds unless times are equal.

Watch out for

• Averaging speeds directly: Students calculate (30 + 60)/2 = 45 km/h for average speed, ignoring that time spent at each speed differs. → Always use Total Distance / Total Time.
• Forgetting unit conversion in train problems: Platform length in metres, speed in km/h — students plug in directly without converting. → Convert km/h to m/s (× 5/18) before using metres in the formula.
• Adding speeds when trains move in the same direction: When one train overtakes another, students add speeds instead of subtracting. → Same direction = subtract; opposite direction = add.

Quick reference

• Speed = Distance / Time; memorize all three forms of this equation.
• km/h to m/s: × 5/18; m/s to km/h: × 18/5.
• Relative speed (opposite): add speeds; (same direction): subtract speeds.
• Average speed = Total Distance / Total Time, never the arithmetic mean of speeds.
• Boat in still water = (Downstream + Upstream)/2; Stream = (Downstream - Upstream)/2.

Check yourself: 3 questions

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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