Fluent adding and subtracting within 100
Fluently add and subtract within 100 using strategies based on place value, properties of operations, and the relationship between addition and subtraction
What a learner can do afterwards
- Add two-digit numbers within 100 efficiently (e.g. 46 + 37 using place-value partitioning)
- Subtract two-digit numbers within 100 fluently (e.g. 83 − 25)
- Choose between strategies (decomposing, compensating, using known facts) based on the numbers involved
1 · Read
You know your facts within 20, and you can add tens and ones within 100. Now you will add and subtract two-digit numbers quickly. Instead of counting one by one, pick a strategy that fits the numbers.
Zoe is trading cards with a friend. She has 57 cards and gets 24 more. Break the numbers into tens and ones: 50 + 20 = 70 and 7 + 4 = 11. Put the parts together: 70 + 11 = 81.
Subtract 72 - 38. Take away 30 first: 72 - 30 = 42. Then take away 8: 42 - 8 = 34.
Look at the numbers before you choose a strategy. If a number sits close to a round number like 30 or 50, use the round number, then fix your answer. If you added too much, take the extra away. If you took away too much, add the extra back. If no number is close to a round number, break both numbers into tens and ones instead.
Break into tens and ones, or round to a nearby ten and fix it: pick whichever strategy fits the numbers.
Original two-digit addition and subtraction questions and reading text authored for the os-taxonomy project.
2 · Watch
Take it off screen
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
19 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.