Primary 3 · Mathematics
P3 time
Most duration marks are lost the same way: subtracting clock times as if an hour had 100 minutes, not 60. Here is why 3:50 to 4:20 is 30 minutes and not 70, and how to count it at home.
- Level
- P3
- Usually taught
- Term 4
- Covers
- 2 concepts
What changes at P3
In Primary 3, time moves from reading a clock to working with it: telling the time to the minute, using the 24-hour clock, and finding how long something lasts. That last one, duration, is where the marks are won or lost.
The catch is that time is not base 10. Sixty minutes make an hour, not a hundred, so the ordinary subtraction that works for money and measurement quietly breaks. The fix is to count on in stages, jumping to the next whole hour first.
The sentence that costs the mark
How long is it from 3:50 to 4:20?
What the working usually does
subtract3:50, 4:20=70 min
Subtracts as if minutes ran to 100, reading 4:20 − 3:50 like 420 − 350. But an hour is 60 minutes, not 100.
What the question asked
count on3:50, 4:20=30 min
Counts on in two hops: 3:50 to 4:00 is 10 minutes, then 4:00 to 4:20 is 20 more. 10 + 20 = 30.
Both answers come from real working, and subtracting is exactly how you find the difference between ordinary numbers, so it feels right for clock times too. But time runs in 60s, not 100s. Cross to the next hour first, then count the rest, and the 70 becomes 30.
| Point | Value |
|---|---|
| Point 1 | 0 |
| Point 2 | 30 |
The same question, worked properly
Do not subtract straight across
an hour = 60 min, not 100Clock minutes do not run up to 100, so a duration is not 4:20 − 3:50 read as 420 − 350. Count on in stages instead.
Count on to the next hour first
3:50 → 4:00 is 10 minThis is the step that fails. Jump to the next whole hour before anything else. From 3:50 that is 10 minutes to 4:00. This is the step the straight-subtraction habit skips, and it is where the mark is lost.
Then count the rest
4:00 → 4:20 is 20 minFrom 4:00 it is a simple 20 minutes on to 4:20, with no hour boundary left to cross.
Add the two hops
10 + 20 = 30 minTen minutes to the hour, then twenty more. The duration is 30 minutes, not the 70 that straight subtraction gives.
How can you tell it has clicked?
Ask how long from 3:50 to 4:20. If they line the times up and subtract to get 70, that is the base-100 slip. If they jump to 4:00 first and then add the rest, they have understood it.
The tell is whether they cross the hour. A child who counts on to the next o'clock, then adds the remaining minutes, is keeping time in 60s. One who subtracts the columns is not.
One thing to try tonight
Use the kitchen clock. It is 3:50, ask how long until 4:20, and count it on the face: jump to 4 o'clock first, then the extra minutes. The jump to the hour is the move that keeps it right.
Whenever a time crosses the hour, pause and ask how many minutes to the next o'clock. That one habit fixes almost every duration slip, and it needs no calculation you cannot do in your head.
What your child will meet in this topic
- 24-Hour Clock
- Duration
Most Singapore schools introduce this in Term 4, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
Counting on in stages is the same method behind timetables and elapsed-time problems in P4 and P5, where durations get longer and cross several hours. The jump-to-the-hour habit scales straight up.
A child who counts time in 60s rarely trips on the 24-hour clock or a bus timetable later. One still subtracting the columns tends to meet the same wall once the problems get longer.
More in Primary 3
Other topics in the same year, so you can see what sits either side of this one.
Struggling with the working rather than the topic itself? How to help your child with problem sums
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.