Mental addition and subtraction (age 6+)
Add and subtract a two-digit number and ones mentally and using concrete/pictorial representations
What a learner can do afterwards
- Calculate 36 + 7 = 43 using objects or mentally
- Calculate 52 − 4 = 48 using a number line or mentally
- Explain bridging through 10 when adding ones to a two-digit number
1 · Read
You already know how to add and subtract, and you know a two-digit number has a tens digit and a ones digit. Now you can add or take away a small number in your head. Most of the time, the tens digit sits still. You only change the ones.
Mia has 52 stickers and gives away 4. She thinks: 52, take away 2 is 50. Take away 2 more is 48. The tens digit dropped from 5 to 4 because she crossed a ten.
Keep the tens number steady in your head. Change only the ones number. If the ones go past 10, or below 0, one ten hops over. That is called bridging through ten.
Add or subtract the ones first, and let a ten hop over only when the ones bridge past ten.
More to read
Add and subtract within 100: FAQ
Adding and subtracting within 100 gets easy when you split numbers into tens and ones. Handle the tens together, handle the ones together, then join the answers. If the ones pass nine, trade ten ones for a new ten. Number lines show each hop clearly.
Intro to place value: FAQ
Place value gives every digit a job based on where it stands. In 53, the 5 means five tens and the 3 means three ones. Tens blocks and ones cubes make this easy to see and touch. Once you read numbers as tens plus ones, adding and subtracting turn into small steps. Adding ten changes only the tens digit.
1.3: Add Whole Numbers (Part 1)
Adding whole numbers means joining counts into one bigger count. Line the digits up by place, start with the ones, and carry tens across when a column passes nine. That carry is just regrouping ten ones as one ten. You can check by adding in a new order, since order does not change the total. Quick recall of facts to 20 makes every bigger sum faster.
1.5: Subtract Whole Numbers (Part 1)
Subtracting whole numbers means finding what is left or how far apart two counts sit. Start with the ones column, and borrow a ten whenever the top digit is too small. Borrowing swaps one ten for ten ones, so the value never changes. Check with addition, because the missing part plus the taken part must rebuild the start.
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2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.