Numbers up to 20
Primary 1 · Mathematics
Numbers up to 20
Lesson 4 of 7
13 min
Let's begin
Ishimwe brings 14 maize cobs into the kitchen.
His mama asks him to pack them in tens.
He fills one sack with 10 cobs. Four cobs will not fit.
Today you will break every teen number into a ten and some ones.
Look at 14. It has two digits.
The first digit is 1. It means 1 ten, one tied bundle.
The second digit is 4. It means 4 ones, four loose sticks.
So 14 is 1 ten and 4 ones.
Careful! Do not say a one and a four. The 1 is a whole bundle of ten.
| Number | Tens | Ones |
|---|---|---|
| 12 | 1 | 2 |
| 15 | 1 | 5 |
| 18 | 1 | 8 |
| 20 | 2 | 0 |
Worked example
build 17.
Step 1 Put 1 bundle of ten sticks on your mat.
Step 2 Put 7 loose sticks beside it.
Step 3 Count on from 10: 11, 12, 13, 14, 15, 16, 17.
You built 17.
Now do it the other way. Take 13 beans.
Push 10 beans into a group. Leave the rest loose.
You have 1 ten and 3 ones left over. That is 13.
Umutoni has 15 stones. Byiringiro has 9 stones. Who has more?
Look at the tens first. 15 has 1 ten. 9 has no ten at all.
So 15 is bigger.
Careful! Byiringiro's 9 looks like a big digit. Ones can never beat a whole bundle.
Now try two teens. Which is bigger, 12 or 18?
Both have 1 ten. The tens are the same, so look at the ones.
8 ones beat 2 ones. So 18 is bigger.
One more. Which is bigger, 19 or 20?
19 has 1 ten. 20 has 2 tens. So 20 is bigger.
Tip
Tens are the boss
Count the bundles first. The one with more bundles wins.
Only if the bundles match do you look at the loose ones.
A big loose digit can never beat a tied bundle.
See a teen number split into a ten and ones.
Do each one, then tap Show answer to check.
How many tens and ones are in 16?
Answer: 1 ten and 6 ones. The 1 is the bundle.
Which is bigger, 11 or 17?
Answer: 17. Both have 1 ten, and 7 ones beat 1 one.
Which is bigger, 8 or 13?
Answer: 13. It has a whole bundle of ten. 8 has none.
Keza has 1 ten and 9 ones. What is her number?
Answer: 19. Write the 1, then the 9.
Take 20 beans. Make groups of ten. How many groups?
Answer: 2 groups, with 0 beans loose. 20 is 2 tens.
What you learned today
Quick check
How many ones are in 18?
