Adding two two-digit numbers
Add and subtract two two-digit numbers using concrete objects, pictorial representations, and mental methods
What a learner can do afterwards
- Calculate 34 + 27 using base-ten blocks or column addition
- Calculate 63 − 28 using a number line or partitioning
- Explain a strategy for adding or subtracting two two-digit numbers
The lesson
You already know how to add tens in your head, like 30 + 20 = 50. Now you will add two two-digit numbers, like 34 + 27. Split each number into tens and ones. Add the tens together, then add the ones together. If the ones add up to ten or more, turn ten of them into one more ten.
Leo adds 48 + 25. Tens: 40 + 20 = 60. Ones: 8 + 5 = 13. That is 1 ten and 3 ones, so he adds the extra ten to 60. 60 + 13 = 73.
Ava works out 63 − 28. Start at 63. Hop back 20 to land on 43. Hop back 8 more to land on 35. So 63 − 28 = 35.
After you add, check the ones digit first. If the ones made ten or more, make sure you moved that extra ten over. Forgetting it is the easiest mistake to make.
Add or subtract the tens, then the ones, and regroup an extra ten when the ones spill over.
Watch it
Where it sits
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.