How to Solve Speed-Time-Distance Problems with Unit Conversion
Master km/h to m/s conversion (5/18, 18/5) for speed-time-distance aptitude problems, with worked examples and practice questions.
Expected Interview Answer
Speed, time and distance are linked by Distance = Speed × Time, and the most common error is mixing units, so the first move is always converting everything to one consistent system before applying the formula.
The standard conversion is km/h to m/s by multiplying by 5/18, and m/s to km/h by multiplying by 18/5 — this factor comes directly from 1000 meters per 3600 seconds simplified. Once every quantity shares the same unit for distance and the same unit for time, D = S × T, S = D/T, and T = D/S can be applied directly without further adjustment. A frequent trap is leaving one value in km/h and another in m/s inside the same equation, which silently produces a wrong answer even though the method looks correct. Always state units explicitly at each step so a mismatch is caught before the final answer is computed.
- One conversion factor (5/18 or 18/5) handles nearly all unit-mismatch problems
- Explicit unit tracking catches setup errors before they reach the final answer
- The same D = S × T relationship works after conversion without extra formulas
- Prevents the single most common mistake in speed-time-distance interviews
AI Mentor Explanation
A bowler’s delivery speed is quoted in km/h on the broadcast graphic, but a physics analysis of ball flight time needs meters per second, since the pitch length is measured in meters and delivery time in seconds. Converting 144 km/h to m/s means multiplying by 5/18, giving 40 m/s, because 144 kilometers is 144,000 meters and one hour is 3600 seconds, and 144000/3600 simplifies to exactly that factor. Mixing the km/h figure directly into a meters-and-seconds equation without converting would make the ball appear to cross the pitch far too slowly. The conversion factor exists precisely because kilometers and hours don’t match the meters and seconds used in the rest of the calculation.
Worked example
Given speed
- 90 km/h
Convert to m/s
- 90 × 5/18 = 25 m/s
Crossing time
- (500+100)/25 = 24 s
Step-by-Step Explanation
Step 1
Identify mismatched units
Check whether speed is in km/h while distance/time given in meters/seconds.
Step 2
Apply the conversion factor
km/h to m/s: multiply by 5/18. m/s to km/h: multiply by 18/5.
Step 3
Recompute with consistent units
Use D = S × T only after every quantity shares the same unit system.
Step 4
Sanity-check the answer
A converted speed and a physically reasonable time/distance should agree in magnitude.
What Interviewer Expects
- Correct recall and derivation of the 5/18 and 18/5 conversion factors
- Recognizing when a problem mixes units before applying any formula
- Applying D = S × T only after full unit consistency
- Catching unit-mismatch traps embedded in the problem statement
Common Mistakes
- Applying D = S × T with speed in km/h and time in seconds without converting
- Multiplying by 18/5 when converting km/h to m/s instead of 5/18
- Forgetting to convert both quantities when only one appears mismatched at first glance
- Rounding the converted speed too early, compounding error in the final answer
Best Answer (HR Friendly)
“Whenever a speed problem gives you km/h but distances or times in meters and seconds, the very first step is converting units so everything matches — multiply km/h by 5 over 18 to get meters per second, or the reverse by 18 over 5. Only once every quantity is in the same unit system do you apply distance equals speed times time. Skipping this step is the single most common reason a correct-looking method still gives the wrong number.”
Follow-up Questions
- Why does the factor 5/18 come from converting km/h to m/s specifically?
- How would you convert miles per hour to meters per second?
- What happens to the conversion factor if the problem uses km/min instead of km/h?
- How do you verify a converted speed is reasonable before using it further?
MCQ Practice
1. Convert 54 km/h to m/s.
54 × 5/18 = 15 m/s.
2. A car travels at 20 m/s. What is its speed in km/h?
20 × 18/5 = 72 km/h.
3. A train 150m long crosses a pole in 10 seconds. Its speed in km/h is?
Speed = 150/10 = 15 m/s; 15 × 18/5 = 54 km/h.
Flash Cards
km/h to m/s conversion factor? — Multiply by 5/18.
m/s to km/h conversion factor? — Multiply by 18/5.
Why does 5/18 work? — 1000 meters per 3600 seconds simplifies to 5/18.
First step in any mixed-unit speed problem? — Convert all quantities to one consistent unit system before applying formulas.