Primary 1 maths · Operations · About 8 minutes

Add or subtract ones and tens mentally without regrouping

Name the size of the units being added, then change only that column.

What you will learn

Models used: number sentences, stories, bar models.

Start with 27. We want to add 10.

The number at the top of each column is the value of one counter in that column.
Start with 27. We want to add 10.
523 + 30 is 553. The hundreds and ones are untouched, because only tens were added.
The number at the top of each column is the value of one counter in that column.
Add one more ten. The other columns stay just as they were.
A digit tells us how many units there are. Its position tells us what size those units are; that is why a zero can matter.
The number at the top of each column is the value of one counter in that column.
No exchange was needed. Take that same group away to return to the starting number.
27 + 10 = 37
A digit tells us how many units there are. Its position tells us what size those units are; that is why a zero can matter.

What did the picture help us see?

No exchange was needed. Take that same group away to return to the starting number.

What is 65 − 10?

?part remaining10part removed

Adding and taking away are a family. Think: 10 + what makes 65?
10 + ? = 65
10+55=65
55+10=65
65−10=55
65−55=10
10 + 55 = 65. So 65 − 10 = 55.
10 + 55 = 65
10+55=65
55+10=65
65−10=55
65−55=10
Use the picture and the steps we have just followed to finish this question.
10+55=65
55+10=65
65−10=55
65−55=10
What is 65 − 10?
55
So 65 − 10 = 55.
65 − 10 = 55
10+55=65
55+10=65
65−10=55
65−55=10
Adding back 10 should bring us to our starting 65.
Put back what we took away. Check: 55 + 10 = 65.
55 + 10 = 65

Changing the wrong column. Adding tens changes the tens digit only. 523 + 30 is 553; the hundreds and ones stay as they are.

Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.

Find the missing partFind a missing part when the whole and one part are known · About 10 minutes
Missing numbers in take-awayFind the starting number when the parts are known · About 10 minutes
Add by combining two setsAdd by combining two sets · About 8 minutes
Subtract by taking awaySubtract by taking away · About 8 minutes
Connect addition and subtraction with number bondsConnect addition and subtraction with number bonds · About 8 minutes
Use +, − and = correctlyUse +, − and = correctly · About 8 minutes