Time, Speed & Distance — The Complete Basics
Master the core Speed = Distance / Time formula, the 5/18 unit-conversion trick, average speed, and proportionality for SSC-CGL quantitative aptitude.
By the PadhoDost Team · 📖 6 min read · Updated 4 August 2026
Part of SSC CGL & Banking prep🧠 The Auto-Rickshaw Meter
Think of an auto meter. The faster the driver goes (speed) and the longer he drives (time), the higher the fare that ticks up (distance). These three are locked together: fix any two and the third is decided. That single relationship is the whole chapter.
Speed tells you how much distance is covered in one unit of time. If a bus covers 60 km in 1 hour, its speed is 60 km/hr. Every question in this chapter is just this idea rearranged, so if you truly own the one formula below, you can solve almost anything.
| Situation | Rule | Meaning |
|---|---|---|
| Time is same | Distance is proportional to Speed | Go twice as fast, cover twice the distance |
| Distance is same | Speed is inversely proportional to Time | Go twice as fast, take half the time |
| Speed is same | Distance is proportional to Time | Drive twice as long, cover twice the distance |
Average Speed — Where Everyone Slips
📝 Going and coming back
A man goes to a town at 40 km/hr and returns along the same road at 60 km/hr.
Wrong instinct: (40 + 60) / 2 = 50 km/hr. This is NOT correct because he spends more time at the slower speed.
Correct method (equal distance): Average = 2xy / (x + y) = (2 x 40 x 60) / (40 + 60).
= 4800 / 100 = 48 km/hr.
So the true average speed is 48 km/hr, not 50.
How to attack any TSD problem
- 1Write down which quantities are given and which is asked.
- 2Convert everything to one consistent unit system (all km/hr, or all m/s).
- 3Pick the right form of Speed = Distance / Time, or use proportionality if one quantity is constant.
- 4For to-and-fro or equal-distance legs, use the 2xy/(x+y) shortcut.
- 5Recheck the unit of your final answer matches what the question asked for.
Remember these
- ✓Speed = Distance / Time is the parent of every formula here.
- ✓km/hr to m/s: multiply by 5/18; m/s to km/hr: multiply by 18/5.
- ✓Average speed = total distance / total time — never a plain average of speeds.
- ✓For equal distances at two speeds, average = 2xy/(x+y) (the harmonic mean).
- ✓When distance is fixed, faster speed means proportionally less time.
⚡ Quick check
A train 150 m long crosses a pole in 15 seconds. What is its speed in km/hr?
Ready to test yourself? 🎯
Lock it in with the practice test for this chapter.
Take the practice test →