Choosing the right strategy
Choose the best tool for each task, such as mental maths, written working, a number line or a ruler, and explain why it fits.
What a learner can do afterwards
- Choose a mental method for 345 + 200 but a written method for 345 + 278
- Select a ruler vs a metre stick based on the object being measured
- Decide when a number line is more useful than base-ten blocks for a given problem
1 · Read
Not every math problem needs the same tool. Before you start, take a second to look at the numbers and think about what will work best: your head, a pencil and paper, or a picture.
Zara sees 250 + 300. She adds the 300 in her head: 250, 350, 450, 550, done. Then she sees 246 + 379. The numbers do not round evenly, so she writes it down and adds column by column so she does not lose track of the carrying.
For measuring, grab a ruler for a pencil but a metre stick for a doorway. For adding, blocks help you see small groups, but hopping on a number line can be quicker for bigger jumps.
Look at the problem first, then choose the tool, mental math, paper, a number line, or the right measuring tool, that fits it best.
More to read
1.2 Add Whole Numbers
Some additions are easy to do in your head. When you add whole hundreds, like 345 + 200, only the hundreds digit changes, so you can say the answer straight away. Harder additions, like 345 + 278, need regrouping in more than one column. For those, a written column method keeps every digit safe.
Key ideas: use compensation to add
Compensation is a clever mental method. You round one number to a friendly one, add, then fix the extra. To add 198, first add 200 and then take 2 away. This works because adding too much on purpose is easy to undo. Mental methods like this shine when the numbers sit close to hundreds. For trickier numbers with regrouping in every column, writing the sum down is the safer plan.
Key ideas: use compensation to subtract
Subtraction has mental tricks too. To subtract 199, first subtract 200 and then add 1 back. The answer stays correct because you undid the extra you took. Choosing tools matters just as much. A small ruler suits a pencil, while a metre stick suits a classroom wall.
Adding and Subtracting Within 100 Using Place Value Strategies (Grade 2)
Place value helps you choose fast methods. Adding tens to any number only changes the tens digit, so 46 + 30 is quick mental work. When both numbers have tricky ones digits that regroup, drawing base-ten blocks or writing columns keeps things clear. Number lines show jumps between numbers and suit counting up or back. Match the strategy to the numbers and you will work faster with fewer slips.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
Learn first
Optional connections
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
17 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.