Primary 1 · Mathematics
P1 length
A crayon that starts at the 2 cm mark and ends at 10 is not 10 cm long. It is one of the commonest ruler slips in P1, and a pencil shows why.
- Level
- P1
- Usually taught
- Term 3
- Covers
- 1 concept
What changes at P1
Primary 1 measures length in centimetres with a ruler. Your child finds how long an object is and which of two things is longer or shorter. They also draw a line of a given length.
The ruler is the new tool, and reading it is where the marks go. An object does not always sit at 0, and when it does not, the number at its end is not its length.
The sentence that costs the mark
A crayon lies along a ruler from the 2 cm mark to the 10 cm mark. How long is the crayon?
What the working usually does
length:read the end=10 cm
Reads the number where the crayon ends, as if it started at 0.
What the question asked
length:10 − 2=8 cm
Sees the crayon starts at 2, so takes the start away from the end.
The ruler was read accurately: the crayon really does end at 10. Rulers are first taught with objects that start at 0, where reading the end is right. The habit then carries over to the one case where it fails.
| Object | Left (cm) | Right (cm) |
|---|---|---|
| Object 1 | 2 | 10 |
The same question, worked properly
Check the start
starts at 2, not at 0This is the step that fails. Look at the left end first. The crayon starts at the 2 cm mark, so the number at its end cannot be its length.
Read the end
ends at 10The right end of the crayon lines up with the 10 cm mark.
Subtract
10 − 2 = 8 cmThe crayon is 8 cm long. Check it by counting the 1 cm spaces between the two ends: there are 8 of them.
How can you tell it has clicked?
Lay a pencil on a ruler starting at the 3 cm mark and ask how long it is. If the answer is the number at the end, the start is being ignored.
A child who looks at both ends first, or counts the 1 cm spaces between them, has it. Counting the marks instead of the spaces gives one too many, so it is worth watching how the counting is done.
One thing to try tonight
Put a pencil on a ruler starting at 2 and ask how long it is. Then slide it back to 0 and read it again.
When the two answers do not match, ask which one is the pencil's length. The pencil did not change, only where it was lying.
What your child will meet in this topic
- Length in cm and m
Most Singapore schools introduce this in Term 3, 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
P2 takes length into metres and asks how much longer one object is than another. Each object has to be measured properly before the two can be compared.
Checking where a measurement starts is a habit worth having early. Every scale your child reads later, on a jug or a weighing machine, rewards looking before reading.
The same topic in other years
It comes back each year and gets harder, so it helps to see what came before and what is coming.
More in Primary 1
Other topics in the same year, so you can see what sits either side of this one.
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.