Primary 1 · Mathematics
P1 numbers to 20
Seventeen is said with the seven first, so it often gets written as 71. A bundle of ten straws and a few loose ones puts the digits back in order.
- Level
- P1
- Usually taught
- Term 2
- Covers
- 1 concept
What changes at P1
In Primary 1 the numbers grow past 10, and a new idea arrives with them: a ten. Eleven to nineteen are each one ten and some ones, which is why every one of them is written with a 1 in front.
Your child fills a ten-frame, or a stick of ten cubes, and counts the ones left over. They also read and write the numbers in words, from eleven to twenty, which school worksheets ask for.
The sentence that costs the mark
Write seventeen in numerals.
What the working usually does
write seventeen:7, then 1=71
Writes the digits in the order the word is heard, seven first.
What the question asked
write seventeen:1 ten, 7 ones=17
Puts the ten down first, then the 7 ones.
Both digits are right and neatly written, just in the order they were heard. It looks like a slip of the pencil. It is a place-value idea that has not landed yet, and it shows up again on 14, 16 and every other teen.
| Place | How many |
|---|---|
| Tens | 1 |
| Ones | 7 |
The same question, worked properly
Say it as tens and ones
seventeen = ten and sevenBefore writing anything, say the number the place-value way: ten and seven.
Write the ten first
1 in the tens placeThis is the step that fails. The ten goes down first, even though "seven" is heard first. Writing what is heard first is how 71 appears.
Then the ones
7 in the ones place: 17Check it: 1 ten and 7 ones is 17. The 71 on the other side would be 7 tens and 1 one.
How can you tell it has clicked?
Say "fourteen" and ask your child to write it. If 41 appears, the digits are going down in the order they were heard.
The better check is to ask how many tens and how many ones. A child who says "1 ten and 4 ones" before writing has understood why 14 starts with a 1. Ask about eleven and twelve too, because they do not sound like "ten and one" at all.
One thing to try tonight
Make a bundle of 10 straws with a rubber band and keep a few loose ones.
Say a teen number and ask your child to build it, then write it. The bundle always goes down first, and so does the 1.
What your child will meet in this topic
- Numbers to 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
Numbers to 100 come next in P1, and the same idea carries every two-digit number: 47 is 4 tens and 7 ones. A child who writes the ten first now already has the idea place value is built on.
Adding within 20 leans on it too. The P1 method is to make ten (9 + 4 is 9 + 1 + 3), which only makes sense once a ten is something your child can see.
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.