Primary 1 · Mathematics
P1 addition and subtraction within 100
Every sum in 38 + 45 = 713 is right, and the answer is still wrong. The 13 ones went into one box, and a bundle of straws shows where they belong.
- Level
- P1
- Usually taught
- Term 3
- Covers
- 2 concepts
What changes at P1
Later in Primary 1, sums grow to 100 and your child meets the column method: tens under tens, ones under ones, adding the ones first.
The new step is regrouping. When the ones make ten or more, ten of them become one ten, which moves to the tens column. Your child's school may call this renaming; it is the same step.
The sentence that costs the mark
Find 38 + 45.
What the working usually does
38 + 45:write 13 below=713
Adds the ones correctly, then writes the whole 13 under the ones.
What the question asked
38 + 45:regroup 10 ones=83
Writes 3 ones and moves the 1 ten to the tens column.
Every fact is right, 8 + 5 = 13 and 3 + 4 = 7, and the columns are neat. The error is in what 13 ones means, and nothing in the working ever says it out loud.
The same question, worked properly
Add the ones
8 + 5 = 13 onesOnes first, always. The ones make 13, which is more than one box can hold.
Regroup ten ones
13 ones = 1 ten 3 onesThis is the step that fails. Write 3 in the ones place, and move the 1 ten to the tens column. Writing the whole 13 below is how 713 appears.
Add the tens
3 + 4 + 1 = 8 tens, so 83Include the regrouped ten. Leaving it behind gives 73, the other common slip on this sum.
How can you tell it has clicked?
Ask for 38 + 45 in columns. An answer of 713 means the 13 ones went into one box. An answer of 73 means the regrouped ten was dropped on the way to the tens.
Then ask your child to say the ones out loud: "13 ones is 1 ten and 3 ones." A child who can say that before writing the 3 knows why the 1 moves.
One thing to try tonight
Make 38 and 45 with bundles of 10 straws and some loose ones, then push the two piles together.
Swap every 10 loose straws for one bundle before counting. There cannot be 13 loose straws left at the end, and there cannot be 13 in one box of the sum either.
What your child will meet in this topic
- Adding Within 100
- Subtracting Within 100
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 the same method to 3 digits, regrouping into the hundreds as well as the tens.
Subtraction mirrors it: when there are too few ones on top, 1 ten is regrouped as 10 ones. By P3 the same move runs across zeros, as in 5003 − 1568.
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.