Primary 1 · Mathematics
P1 addition and subtraction within 10
One counting habit makes a P1 child's sums come out one short, every time. It is easy to spot once you know where to look, and a flight of stairs fixes it.
- Level
- P1
- Usually taught
- Term 1
- Covers
- 2 concepts
What changes at P1
Primary 1 takes numbers your child already knows and turns them into sums. They learn number bonds: 6 and 3 make 9, two parts and a whole. Then comes counting on, which means starting from the greater number instead of counting everything from 1.
Subtraction arrives in the same term, as taking away and as finding a missing part. One bond gives four facts, 6 + 3, 3 + 6, 9 − 3 and 9 − 6, and schools call the set a fact family.
The sentence that costs the mark
Count on to find 6 + 3.
What the working usually does
6 + 3:6, 7, 8=8
Says 6 as the first count, so three counts only reach 8.
What the question asked
6 + 3:7, 8, 9=9
Holds the 6 and says the next number first, so three counts land on 9.
Both are three careful counts on three fingers, and 8 is only one away from 9, so it reads as a slip. It is not a slip. The same habit lands one short on every sum, and it keeps doing so until someone spots where the counting starts.
| Point | Value |
|---|---|
| Point 1 | 6 |
| Point 2 | 9 |
The same question, worked properly
Start at the greater number
start at 6Hold 6 in your head. It is where the count starts from, not the first number counted.
Count on three
7, 8, 9This is the step that fails. The first count is 7, the number after 6. Say 6 here instead and the count runs 6, 7, 8, stopping one short with working that still looks careful.
Check with the number bond
6 + 3 = 96 and 3 make 9. The bond agrees with the count, which is how a child can check their own counting without starting again.
How can you tell it has clicked?
Ask for 6 + 3 and watch the fingers, not the answer. If the first finger goes up on "6", the count will land on 8 however carefully the rest is done.
A right answer can hide it too. A child who counts all nine from 1 gets 9 without counting on at all. The step this term asks for is holding the 6 and saying "7" first.
One thing to try tonight
Walk it on the stairs. Stand on step 6, then climb 3 steps, saying one number each time a foot lands: 7, 8, 9.
Standing still and saying "6" is the mistake, and on the stairs it feels wrong straight away, because nobody has moved yet. Two or three goes usually make the rule stick: the number you stand on is never counted.
What your child will meet in this topic
- Adding Within 10
- Subtracting Within 10
Most Singapore schools introduce this in Term 1, 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 carries the rest of P1. Within 20, your child learns to make ten (7 + 6 is 7 + 3 + 3), and that only works once counting from a number, rather than from 1, feels natural.
The same off-by-one comes back with bigger numbers, as 28 + 3 counted "28, 29, 30". A child who holds the first number now carries the habit into numbers to 100 without relearning it.
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.