7 Speed, Distance & Time Mistakes That Cost PSLE Marks

Speed, Distance & Time questions appear in almost every PSLE Paper 2 — and they’re among the highest mark-losers. Most students know the formula but still lose marks to the same 7 traps. Fix these, and you could rescue 8-12 marks on exam day.

Speed, Distance & Time (SDT) is one of the most feared topics in PSLE Mathematics. It combines formulas, unit conversions, time arithmetic, and multi-step reasoning — all in a single question.

The problem? Even students who memorise Speed = Distance ÷ Time still get caught by sneaky traps that examiners love to set.

Here are the 7 most common SDT mistakes we see P5-P6 students make, ranked from most damaging to least.

The SDT Triangle: Your Starting Point

Before we dive into the mistakes, make sure you know the three core formulas. The SDT Triangle is the fastest way to remember them:

Want to find	Formula
Speed	Speed = Distance ÷ Time
Distance	Distance = Speed × Time
Time	Time = Distance ÷ Speed

💡 The SDT Triangle Trick

Draw a triangle with D on top, S on the bottom-left, and T on the bottom-right. Cover the letter you want to find — what’s left is the formula!

Mistake 1: Average Speed ≠ Average of Two Speeds

This is the #1 speed killer in PSLE. Students add two speeds and divide by 2. It looks logical — but it’s almost always wrong.

The Average Speed Trap

Question:

Amir drove from Town A to Town B at 60 km/h. He drove back from Town B to Town A at 40 km/h. What was his average speed for the whole journey?

Wrong:

Average speed = (60 + 40) ÷ 2 = 50 km/h ✗

Student averaged the two speeds — this only works if the TIME for each leg is equal, not the distance!

Why it’s wrong: Amir spent more time driving at 40 km/h (slower leg) than at 60 km/h (faster leg). The slower speed “weighs” more.

Correct Method:

Let the one-way distance = 120 km (LCM of 60 and 40)

Time for A→B = 120 ÷ 60 = 2 h

Time for B→A = 120 ÷ 40 = 3 h

Total distance = 120 + 120 = 240 km

Total time = 2 + 3 = 5 h

Average speed = 240 ÷ 5 = 48 km/h ✓

⚠️ Golden Rule

Average speed = Total Distance ÷ Total Time. Always. No exceptions. Never add speeds and divide by 2 unless the question tells you the time for each leg is equal.

Mistake 2: Forgetting That Rest Time Counts

When a question says “Ali stopped for a 30-minute lunch break”, many students subtract the break from the total time. But if the question asks for average speed for the whole journey, rest time is included!

The Rest Stop Trap

Question:

Siti cycled 24 km in 2 hours, rested for 30 minutes, then cycled another 12 km in 1 hour. Find her average speed for the whole journey.

Wrong:

Total time = 2 + 1 = 3 h (forgot the rest!)

Average speed = 36 ÷ 3 = 12 km/h ✗

Correct:

Total distance = 24 + 12 = 36 km

Total time = 2 h + 0.5 h + 1 h = 3.5 h

Average speed = 36 ÷ 3.5 = $\frac{36}{\frac{7}{2}} = 36 \times \frac{2}{7} = \frac{72}{7}$ = $10\frac{2}{7}$ km/h ✓

💡 Quick Check

Read the question twice. If it says “average speed for the whole journey” → include ALL time (rest, waiting, lunch). If it says “average speed while moving” → exclude rest time. PSLE almost always means the whole journey.

Mistake 3: Time Arithmetic Blunders

Time doesn’t work like decimals. 1 hour 45 minutes ≠ 1.45 hours. This is one of the most common careless errors in SDT.

The Decimal Time Trap

Question:

A bus left at 9.15 a.m. and arrived at 11.45 a.m. How long was the journey?

Wrong:

11.45 − 9.15 = 2.30 → “2 hours 30 minutes” → writes 2.30 h in formula ✗

2.30 hours means 2 hours and 0.30 × 60 = 18 minutes, NOT 30 minutes!

Correct Conversion:

Journey time = 2 h 30 min = 2 h + $\frac{30}{60}$ h = 2.5 h ✓

Or express as a fraction: $2\frac{1}{2}$ h = $\frac{5}{2}$ h ✓

Common time-to-decimal conversions to memorise:

Minutes	As decimal	As fraction
10 min	0.167 h	$\frac{1}{6}$ h
15 min	0.25 h	$\frac{1}{4}$ h
20 min	0.333 h	$\frac{1}{3}$ h
30 min	0.5 h	$\frac{1}{2}$ h
45 min	0.75 h	$\frac{3}{4}$ h

💡 Pro Tip: Use Fractions

In PSLE, use fractions instead of decimals for time. $\frac{3}{4}$ h is exact, while 0.75 h is also fine — but 0.45 h for 45 minutes is WRONG. When in doubt, convert minutes to a fraction over 60.

Mistake 4: Mixing Up Units (km/h vs m/min)

PSLE loves giving you speed in one unit and distance or time in another. If you don’t convert, you’ll get a completely wrong answer.

The Unit Mismatch Trap

Question:

A car travels at 90 km/h. How far does it travel in 40 minutes?

Wrong:

Distance = 90 × 40 = 3600 km ✗

Student used 40 minutes directly with km/h — units don’t match!

Correct:

Step 1: Convert 40 min to hours → $\frac{40}{60} = \frac{2}{3}$ h

Step 2: Distance = Speed × Time = $90 \times \frac{2}{3}$ = 60 km ✓

⚠️ Unit-Match Checklist

Before plugging into any SDT formula, check: Are ALL three quantities in matching units?

Speed in km/h → Distance in km, Time in hours
Speed in m/min → Distance in m, Time in minutes
Speed in m/s → Distance in m, Time in seconds

Quick conversion shortcuts:

Convert	Method	Example
km/h → m/min	÷ by 0.06 (or × 1000 ÷ 60)	6 km/h = 100 m/min
m/min → km/h	× 0.06 (or × 60 ÷ 1000)	200 m/min = 12 km/h
km → m	× 1000	3.5 km = 3500 m
min → h	÷ 60	45 min = 0.75 h

Mistake 5: Finding the Wrong “Time” in Departure/Arrival Questions

Many PSLE problems give you departure and arrival times and ask you to find speed. Students often subtract the times incorrectly — especially when crossing the hour mark.

The Clock Arithmetic Trap

Question:

A train left Station X at 10.50 a.m. and arrived at Station Y at 1.20 p.m. The distance between the stations is 150 km. Find the speed of the train.

Wrong:

Time = 1.20 − 10.50 = … (student gets confused and writes 2.70 h or 3.30 h) ✗

Correct Method: Count in steps!

Step-by-step:

10.50 a.m. → 11.00 a.m. = 10 min

11.00 a.m. → 1.00 p.m. = 2 h

1.00 p.m. → 1.20 p.m. = 20 min

Total = 2 h 30 min = $2\frac{1}{2}$ h

Speed = 150 ÷ $\frac{5}{2}$ = $150 \times \frac{2}{5}$ = 60 km/h ✓

💡 The Bridge Method

Never subtract times directly (e.g., 1.20 − 10.50). Instead, bridge to the next round hour first, count the full hours, then add the remaining minutes. This eliminates clock arithmetic errors.

Mistake 6: Using the Wrong Formula Rearrangement

Students who learn Speed = Distance ÷ Time sometimes rearrange it incorrectly when solving for Distance or Time.

The Formula Swap Trap

Question:

A cyclist travels at 15 km/h. How long does she take to cover 45 km?

Wrong:

Time = Speed ÷ Distance = 15 ÷ 45 = $\frac{1}{3}$ h ✗

Student swapped the numerator and denominator!

Correct:

Time = Distance ÷ Speed = 45 ÷ 15 = 3 h ✓

💡 Sense-Check Your Answer

Always do a quick sense check: at 15 km/h, can you really cover 45 km in just 20 minutes ( $\frac{1}{3}$ h)? No way! That’s barely enough time to cycle 5 km. The answer should be bigger than 1 hour — so 3 hours makes sense.

Memory aid for the SDT Triangle:

$\text{D} = \text{S} \times \text{T} \qquad \text{S} = \frac{\text{D}}{\text{T}} \qquad \text{T} = \frac{\text{D}}{\text{S}}$

Notice: D is always on top (in the numerator or being multiplied). If your answer has Distance on the bottom, you’ve swapped the formula.

Mistake 7: Not Answering What the Question Asks

This is the ultimate frustration — doing all the math correctly but giving the wrong final answer because you stopped one step too early (or answered in the wrong form).

The Last-Step Trap

Question:

Car A left Town X at 8 a.m. travelling at 80 km/h. Car B left Town X at 9 a.m. travelling at 100 km/h on the same route. At what time did Car B catch up with Car A?

Wrong:

Student finds that Car B takes 4 hours to catch up and writes: 4 hours ✗

The question asked “at what time”, not “how long”!

Correct Working:

In 1 h head start, Car A travels: 80 × 1 = 80 km ahead

Car B gains: 100 − 80 = 20 km/h on Car A

Time for B to close 80 km gap: 80 ÷ 20 = 4 h

Car B left at 9 a.m., so catch-up time = 9 a.m. + 4 h = 1 p.m. ✓

⚠️ Final Answer Checklist

Before writing your answer, re-read the question’s last line:

“At what time” → give a clock time (e.g., 1 p.m.)
“How long” → give a duration (e.g., 4 hours)
“How far” → give a distance with units (e.g., 320 km)
“What speed” → give speed with units (e.g., 48 km/h)

Practice Tool: Check Your SDT Calculations

Use our Speed, Distance & Time calculator to verify your working. Enter any two values to find the third — with step-by-step working shown.

Speed, Distance & Time Calculator

Cover what you want to find. The remaining shows the formula!

Cover S → D ÷ T

What do you want to calculate?

Distance

Time

Quick-Reference: The 7 Mistakes at a Glance

#	Mistake	Fix
1	Average of two speeds	Use Total Distance ÷ Total Time
2	Ignoring rest time	Include ALL time in average speed
3	45 min = 0.45 h	Convert minutes to fractions: $\frac{45}{60} = \frac{3}{4}$ h
4	Mixing km/h with minutes	Match ALL units before calculating
5	Subtracting clock times directly	Bridge to the nearest hour first
6	Distance ÷ Speed swapped	D is always on TOP in the formula
7	Answering “how long” instead of “what time”	Re-read the last line of the question

The 30-Second SDT Exam Checklist

Before submitting any speed question, run through this:

Units match? Speed, distance, and time all in the same system?
Time converted correctly? Minutes turned into fractions (not decimals that look like minutes)?
Rest time included? If the question says “whole journey”, include stops.
Formula correct? D always on top — $S = \frac{D}{T}$ , $T = \frac{D}{S}$ .
Answer matches the question? Time vs clock time vs distance vs speed?
Sense check? Does the number feel reasonable?

💡 Top Scorer Habit

Students who consistently score AL1 in PSLE Math spend the last 2-3 minutes of every speed question running this checklist. It takes 30 seconds and can save you 4-5 marks on a single question.

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7 Speed, Distance & Time Mistakes That Cost PSLE Marks

7 Speed, Distance & Time Mistakes That Cost PSLE Marks

The SDT Triangle: Your Starting Point

Mistake 1: Average Speed ≠ Average of Two Speeds

Mistake 2: Forgetting That Rest Time Counts

Mistake 3: Time Arithmetic Blunders

Mistake 4: Mixing Up Units (km/h vs m/min)

Mistake 5: Finding the Wrong “Time” in Departure/Arrival Questions

Mistake 6: Using the Wrong Formula Rearrangement

Mistake 7: Not Answering What the Question Asks

Practice Tool: Check Your SDT Calculations

Speed, Distance & Time Calculator

Quick-Reference: The 7 Mistakes at a Glance

The 30-Second SDT Exam Checklist

Ready to Master Speed Problems?

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