Primary 1 · Mathematics
P1 numbers to 100
Ask which is greater, 39 or 41, and the 9 can win. The ones digit catches the eye, and a pile of coins and buttons settles it.
- Level
- P1
- Usually taught
- Term 2
- Covers
- 3 concepts
What changes at P1
In the second half of Primary 1 your child counts to 100 and writes each number in numerals and in words (47 is forty-seven, with a hyphen). They order sets of numbers both ways, and meet their first number patterns: counting on and back in 2s, 5s and 10s, and the missing number in 35, 40, ___, 50.
Underneath all of it is one idea: a two-digit number is tens and ones. 39 is 3 tens and 9 ones, and comparing two numbers means comparing the tens first.
The sentence that costs the mark
Which is greater, 39 or 41?
What the working usually does
compare:the ones: 9 > 1=39
Looks at the last digit of each number, and 9 is bigger than 1.
What the question asked
compare:the tens: 4 > 3=41
Looks at the tens first, and 4 tens is more than 3 tens.
"9 is bigger than 1" is true, and the 9 is the digit the eye lands on last. Real digits were compared, just in the wrong place, so the answer comes with a reason that sounds right.
| Place | How many |
|---|---|
| Tens | 3 |
| Ones | 9 |
| Place | How many |
|---|---|
| Tens | 4 |
| Ones | 1 |
The same question, worked properly
Read them as tens and ones
39 = 3 tens 9 ones, 41 = 4 tens 1 oneSay each number the place-value way before comparing anything.
Compare the tens first
4 tens > 3 tensThis is the step that fails. The tens decide. Nine loose ones can never make up an extra ten, because ten of them would already be one more ten.
So 41 is greater
41 > 39There is no need to look at the ones. They decide only when the tens are the same, as in 43 and 47.
How can you tell it has clicked?
Ask which is greater, 39 or 41. If the answer is 39, ask why. "Because 9 is bigger" means the ones were compared first.
A child who says "41, because 4 tens is more than 3 tens" has it. Then try 43 and 47, where the tens are the same, to check they know when the ones do matter.
One thing to try tonight
Use 10-cent coins as tens and small buttons as ones. Build 39 and 41, and ask which pile is worth more.
The pile of nine buttons looks like a lot, which is exactly the point. However the buttons are spread out, nine of them never beat one extra 10-cent coin.
The coins also do number patterns. Lay out 35, 40, a gap, then 50, and ask what each step adds. Once your child says "5 more each time", counting on from 40 fills the gap with 45.
What your child will meet in this topic
- Numbers to 100
- 2-Digit Place Value
- Number Patterns
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
P2 takes numbers to 1000, and the rule simply moves one place to the left: compare the hundreds first, then the tens, then the ones.
Tens and ones also carry adding and subtracting within 100 later in P1, where the column method regroups ten ones into a ten.
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.