Primary 1 · Mathematics
P1 addition and subtraction within 20
Put a box after 9 + 6 = and many P1 children write 15, even when more follows. The equals sign means "is the same as", and a coat-hanger balance makes that visible.
- Level
- P1
- Usually taught
- Term 2
- Covers
- 2 concepts
What changes at P1
In the second half of Primary 1, sums grow to 20 and two methods arrive. Making ten: 8 + 5 is 8 + 2 + 3, so 10 + 3. Subtracting from ten: 13 − 5 is 10 − 5, plus the 3.
The equals sign changes too. Until now it always sat just before the answer. From here, questions put working on both sides of it, and your child has to read it as "is the same as".
The sentence that costs the mark
What number goes in the box? 9 + 6 = + 5
What the working usually does
the box:9 + 6=15
Adds the left side and writes the total straight into the box.
What the question asked
the box:15 − 5=10
The left makes 15, so □ + 5 must make 15 as well.
9 + 6 = 15 is a correct sum, written down neatly. Every sum before this one put the answer right after the equals sign, so the habit feels like knowledge. Here the box is only part of the other side.
| Circle | Number |
|---|---|
| Circle 1 | 9 |
| Circle 2 | 6 |
| Circle 3 | unknown |
| Circle 4 | 5 |
The same question, worked properly
Work out the side you can
9 + 6 = 15The left side is complete, so find its total first.
Make the other side the same
+ 5 must also make 15This is the step that fails. The equals sign means both sides hold the same amount. Writing 15 in the box would make the right side 20, which is where it goes wrong.
Find the missing number
15 − 5 = 10, so = 10What and 5 make 15? Ten. Check it: 10 + 5 = 15, the same as 9 + 6.
How can you tell it has clicked?
Write 9 + 6 = □ + 5 and ask what goes in the box. If the answer is 15, the equals sign is being read as "the answer comes next".
Then ask your child to read the sentence aloud, saying "is the same as" for the equals sign. Hearing "9 and 6 is the same as something and 5" is often enough for them to fix the box on their own.
One thing to try tonight
Hang two paper cups from a coat hanger to make a balance. Put 9 buttons and then 6 more in one cup, and 5 in the other.
Ask how many more buttons the second cup needs to hang level. Added one at a time, the answer comes out as 10, never 15, and the hanger shows why.
What your child will meet in this topic
- Adding Within 20
- Subtracting Within 20
Most Singapore schools introduce this in Term 2, 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
Making ten carries straight into numbers to 100, where 38 + 7 is worked as 38 + 2 + 5.
Reading = as "is the same as" matters for years: every missing-number question, and P6 algebra after them, rests on it. A child who sees a balance now does not have to unlearn a habit later.
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.