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
The lesson
You already know how to add and subtract numbers up to 100. Now it is time to get quick at it. Instead of counting one by one, pick a strategy that fits the numbers in front of you.
Add 46 + 37. Break each number into tens and ones: 40 + 30 = 70, and 6 + 7 = 13. Put the parts back together: 70 + 13 = 83.
Subtract 83 - 25. Take away the tens first: 83 - 20 = 63. Then take away the extra ones: 63 - 5 = 58.
Look at the numbers before you choose a strategy. If a number sits close to a round number like 30 or 50, add or subtract the round number, then fix your answer by a little. If the ones digits add past 10, 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.
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.