RRB Group D · Bite-size lessons

Time and Distance

About 6 min · Mathematics · English

Time worked out from the lesson's 420 words at 150 words a minute, plus a minute for each question.

The questions test your ability to connect speed, time and distance using the fundamental relationship: Distance = Speed × Time. Mastery of this topic is non-negotiable because it directly links to real-world railway operations—trains meeting, overtaking, crossing platforms, and relative motion.

Key ideas

• Fundamental Relationship: Distance = Speed × Time. Rearrange to find any one quantity if two are known: Speed = Distance/Time, Time = Distance/Speed.
• Unit Conversion: 1 km/h = 5/18 m/s and 1 m/s = 18/5 km/h. Always check units in word problems and convert as needed before calculation.
• Relative Speed (same direction): When two objects move in the same direction, their relative speed = |Speed₁ - Speed₂|. Used when one overtakes the other.
• Relative Speed (opposite direction): When two objects move towards each other, their relative speed = Speed₁ + Speed₂. Used when they meet or cross.
• Average Speed: For a journey with different speeds over equal distances, Average Speed = Total Distance / Total Time, NOT the arithmetic mean of speeds.

Watch out for

• Mixing units: Calculating distance in km but time in seconds without converting speed to m/s. Always ensure all quantities are in compatible units before applying formulas.
• Wrong relative speed direction: Adding speeds when objects move in the same direction (should subtract), or subtracting when they move opposite (should add). Remember: opposite → add, same → subtract.
• Averaging speeds incorrectly: Taking the arithmetic mean (S₁ + S₂)/2 instead of using the harmonic mean formula (2S₁S₂)/(S₁ + S₂) for equal distances. Average speed depends on time spent, not just speeds.

Quick reference

• Distance = Speed × Time — The mother formula. Rearrange as needed.
• 1 km/h = 5/18 m/s; 1 m/s = 18/5 km/h — Memorize these conversions cold.
• Relative speed: opposite → add; same → subtract — Critical for train and meeting problems.
• Train crossing: add lengths — Train + platform or train + train.
• Boats: downstream = boat + stream; upstream = boat - stream — Then solve for unknowns.

Check yourself: 3 questions

This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 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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