Speed, Distance & Time: The PSLE Math Topic That Trips Everyone Up
Why do Speed problems feel so confusing? Learn the simple triangle trick and real-world strategies that make SDT problems click.
Speed, Distance & Time: The PSLE Math Topic That Trips Everyone Up
“If a car travels at 60 km/h for 2 hours, how far does it go?”
Simple, right? But what if the same car drives at 40 km/h for the first half and 80 km/h for the second half - what’s its average speed?
Spoiler: It’s NOT 60 km/h. And that surprise is exactly why Speed, Distance, and Time trips up so many students.
Why SDT Feels So Hard
Here’s the truth: Speed, Distance, and Time problems aren’t actually difficult math. The calculations are usually just multiplication and division. So why do students freeze when they see these questions?
⚠️ The real challenge is knowing WHICH operation to use.
Should you multiply or divide? Do you divide distance by time, or time by distance? In the heat of an exam, it’s easy to mix them up.
That’s where the SDT Triangle comes in - a simple visual trick that tells you exactly what to do, every single time.
The SDT Triangle: Your New Best Friend
Imagine a triangle with three parts. At the top sits Distance (D). At the bottom left is Speed (S), and at the bottom right is Time (T).
How to Use It:
- 1Cover what you want to find.
- 2What’s left tells you the formula.
Finding Distance?
Cover D, you see S x T
Finding Speed?
Cover S, you see D / T
Finding Time?
Cover T, you see D / S
Making It Real: A Quick Mental Check
Before we dive into practice problems, here’s a sanity check that will save you from silly mistakes:
💡 The 'Does This Make Sense?' Test
- Faster speed + same time = MORE distance (obviously!)
- Same distance + faster speed = LESS time (you arrive sooner)
- Same distance + slower speed = MORE time (takes longer)
If your answer violates common sense (like a car taking 10 hours to drive 5 km), you’ve probably mixed up your operations. The triangle never lies!
Let’s Practice: 3 Problems, 3 Patterns
Problem 1: Finding Distance
A bus travels at 45 km/h for 3 hours. How far does it travel?
Solution:
- Step 1: What do we want? Distance
- Step 2: Cover D in the triangle → we see S x T
- Step 3: Calculate:
Answer: 135 km
Problem 2: Finding Speed
Sarah cycles 24 km in 2 hours. What is her speed?
Solution:
- Step 1: What do we want? Speed
- Step 2: Cover S in the triangle → we see D / T
- Step 3: Calculate:
Answer: 12 km/h
Problem 3: Finding Time
A train travels at 80 km/h. How long does it take to travel 200 km?
Solution:
- Step 1: What do we want? Time
- Step 2: Cover T in the triangle → we see D / S
- Step 3: Calculate:
Answer: 2.5 hours (or 2 h 30 min)
The PSLE Trap: Average Speed
Remember that tricky question from the beginning? Here’s why average speed catches students off guard:
❌ Common Mistake Alert!
“A car drives 40 km/h for the first half and 80 km/h for the second half. The average speed must be 60 km/h, right?”
WRONG!
You can’t just average speeds. You must use the golden rule:
Average Speed = Total Distance / Total Time
Let’s Work It Out:
Say the car travels 120 km total (60 km at each speed):
- First 60 km at 40 km/h → Time = 60 / 40 = 1.5 hours
- Second 60 km at 80 km/h → Time = 60 / 80 = 0.75 hours
- Total time = 1.5 + 0.75 = 2.25 hours
- Average speed = 120 / 2.25 = 53.3 km/h
The car spends more time at the slower speed, so the average gets pulled down below 60 km/h. This is a classic PSLE trick!
Unit Conversions: Don’t Get Caught!
PSLE loves mixing units. Here are the conversions you MUST know:
Time Conversions
- 1 hour = 60 minutes
- 1 minute = 60 seconds
- 30 minutes = 0.5 hours
- 15 minutes = 0.25 hours
- 45 minutes = 0.75 hours
Distance Conversions
- 1 km = 1000 m
- 1 m = 100 cm
- km/h to m/min: / 60 x 1000
- or simply: x 16.67
💡 Pro Tip
Before you calculate, check that your units match! If speed is in km/h, time should be in hours (not minutes). Convert first, calculate second.
Your SDT Survival Kit
- ✓Use the SDT Triangle - Cover what you need, use what’s left
- ✓Check your units - Convert to matching units before calculating
- ✓Average Speed trap - Always use Total Distance / Total Time
- ✓Sanity check - Does your answer make real-world sense?
- ✓Practice the patterns - Most SDT questions follow the same 3 types
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