Common Mistakes with Speed Distance Time in GCSE Maths

You remember the formula, enter the numbers and still lose marks. Often, common mistakes with speed distance time in GCSE Maths come from mismatched units, decimal time or choosing the wrong calculation. The reliable method is to identify the quantity you need, make the units consistent, write the formula and check whether the answer makes sense.

Imagine a student checking a marked practice paper. Their arithmetic is mostly correct, but several journey questions have gone wrong. Following those errors shows how small changes to your working can protect marks.

A checklist before you calculate

For every speed, distance and time question:

• Identify whether you need speed, distance or time.
• Write down the given values with their units.
• Convert units to match the answer required.
• Substitute into the correct formula.
• Show your working and include the final unit.

The three relationships are:

$$speed=\frac{distance}{time}$$ $$distance=speed\times time$$ $$time=\frac{distance}{speed}$$

MathsGenie's speed and density revision guide explains these relationships. The key idea is that “per” means division: kilometres per hour tells you how many kilometres are travelled in each hour.

Mistake: using minutes with kilometres per hour

Our hypothetical student's first answer uses the right formula but incompatible units.

Question: A cyclist travels $18km$ in $45$ minutes. Calculate their average speed in $km/h$.

The mistake: Dividing $18$ by $45$ and labelling the answer $km/h$.

Why it happens: The student sees distance and time and immediately divides. However, using minutes gives a speed in kilometres per minute.

The correct method: Convert the time into hours first.

$$45minutes=\frac{45}{60}hours=0.75hours$$

Then calculate the speed:

$$speed=\frac{18}{0.75}=24km/h$$

Check the answer against the journey: travelling $18km$ in less than an hour means the hourly speed must be greater than $18km/h$.

Once you have this units-first method, practise applying it through MathsGenie's speed and density Lessons and Question Bank before mixing different question types.

Mistake: treating clock time as decimal time

The next correction looks similar, but the misunderstanding is different.

Question: A car travels at an average speed of $60km/h$ for $1$ hour $30$ minutes. Find the distance travelled.

The mistake: Using $1.30$ hours in the calculation.

Why it happens: A clock separates hours and minutes, so writing the same digits after a decimal point feels natural. But an hour contains $60$ minutes, while decimal notation uses hundredths.

The correct method: Convert only the minutes part into a fraction of an hour.

$$time=1+\frac{30}{60}=1.5hours$$

Now multiply speed by time:

$$distance=60\times 1.5=90km$$

The same issue appears when converting an answer back. If your calculator gives $1.4$ hours, the decimal part represents:

$$0.4\times 60=24minutes$$

So the time is $1$ hour $24$ minutes. Keep decimal hours for calculations, then convert to hours and minutes if requested.

Mistake: multiplying when you need to divide

After fixing the units, our student notices another error: using the distance calculation when the question asks for time.

Question: A runner covers $1500m$ at an average speed of $5m/s$. How long does the run take?

The mistake: Calculating $1500\times 5$.

Why it happens: Recognising the topic can trigger a remembered operation before the student identifies the unknown.

The correct method: Write the required quantity first.

$$time=\frac{distance}{speed}$$ $$time=\frac{1500}{5}=300s$$

If the question asks for minutes:

$$time=\frac{300}{60}=5minutes$$

A useful check is to consider what a greater speed would do. For a fixed distance, travelling faster should take less time. Dividing distance by speed matches that relationship.

Mistake: averaging the two speeds

This error can survive even when every unit is correct.

Question: A car travels $60km$ at $30km/h$, then another $60km$ at $60km/h$. Find its average speed for the whole journey.

The mistake: Taking the arithmetic mean:

$$\frac{30+60}{2}=45km/h$$

Why it happens: “Average” often suggests adding values and dividing by how many there are. But average speed depends on the total distance and total time. The car spends longer travelling at the slower speed.

The correct method: Calculate each journey time first.

$$first\; time=\frac{60}{30}=2hours$$ $$second\; time=\frac{60}{60}=1hour$$

Then use the totals:

$$average\; speed=\frac{60+60}{2+1}=40km/h$$

The arithmetic mean of two speeds works when the time spent at each speed is equal. Using total distance divided by total time is the dependable general method. For an entire journey that includes a stop, include the stopping time unless the question specifies moving time only.

Mistake: reading distance as speed on a graph

A final error appears in a distance-time graph question. Our student reads the vertical coordinate and reports it as the speed.

Why it happens: The graph describes movement, so a higher point can feel like a faster journey. In fact, the vertical axis shows distance. Speed comes from the gradient.

For a straight section:

$$speed=\frac{change\; in\; distance}{change\; in\; time}$$

Worked correction: Suppose distance increases from $20m$ to $80m$ between $4s$ and $14s$.

$$speed=\frac{80-20}{14-4}=\frac{60}{10}=6m/s$$

Use changes in both quantities. Dividing the coordinates of the endpoint only works when the relevant straight section passes through the origin.

A horizontal section represents no change in distance, so the object is stationary. Revisit MathsGenie's distance-time graph explanation if reading gradients is the part that slows you down.

Exam-style practice: check what changed

Try these without looking back at the examples. Write a formula and a unit for each answer.

• Cycling: A cyclist travels $15km$ in $50$ minutes. Find the average speed in $km/h$.
• Driving: A car travels at $72km/h$ for $1$ hour $20$ minutes. Find the distance travelled.
• Two stages: A vehicle travels $40km$ at $40km/h$, then $40km$ at $80km/h$. Find its overall average speed to one decimal place.

Answers and checks

Cycling: Convert minutes to hours before dividing.

$$speed=15÷\frac{50}{60}=18km/h$$

Driving: Convert the minutes part, then multiply.

$$distance=72\times \left(1+\frac{20}{60}\right)=96km$$

Two stages: Find the total time before calculating average speed.

$$total\; time=\frac{40}{40}+\frac{40}{80}=1.5hours$$ $$average\; speed=\frac{80}{1.5}\approx 53.3km/h$$

For further graph practice, use the distance-time graphs question booklet. When marking your work, identify the first incorrect step. A unit error needs different practice from a formula error.

Turn corrections into reliable marks

Our hypothetical student's useful discovery is that each wrong answer has a specific cause they can fix. Your next step is to make those corrections routine when the wording changes and the questions are mixed. Start fixing your speed distance time mistakes with MathsGenie's Lessons and Question Bank.