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
The lesson
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.
Watch it
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 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.