Mental and written addition and subtraction
Solve addition and subtraction problems using mental and written methods, including problems involving numbers, quantities, and measures
What a learner can do afterwards
- Solve a two-step problem: 'I had 35p, spent 12p, then found 5p. How much now?'
- Choose an appropriate method (mental or written) for a given problem
- Solve problems involving measures, e.g. 'a ribbon is 45cm, I cut off 18cm'
1 · Read
Real problems do not always look like a plain sum. A price, a ribbon, or a bag of flour can hide an addition or a subtraction inside it. Read the problem carefully: are things going up, going down, or both?
A ribbon is 45cm long. Priya cuts off 18cm to wrap a gift. How long is it now? Subtract, because a piece is taken away: 45cm - 18cm = 27cm.
Round, easy numbers like 50 - 20 are quick to do in your head. Numbers with borrowing, like 84 - 37, are safer to write down in columns. Whichever way you pick, check your answer by working backwards.
Read the problem to see if things are joining or leaving, then add or subtract, using your head for easy numbers and paper for tricky ones.
More to read
Adding and subtracting within 1,000: FAQ
Big numbers are just small numbers with more places. Add or subtract each place on its own, starting from the ones, and carry or borrow across places as needed. Saying the number in hundreds, tens, and ones keeps every digit in its home. Estimate first to tell if your exact answer lands nearby. Word problems with money or measures work the same way.
Adding and Subtracting Within 100 Using Place Value Strategies (Grade 2)
Place value splits numbers into tens and ones, which makes adding and subtracting within 100 much easier. Add the tens first, then the ones, and join the parts at the end. Breaking numbers apart this way keeps every step small and safe. Draw blocks or a number line if the picture helps. The same splitting trick works for take away problems too.
1.3: Add Whole Numbers (Part 1)
Adding whole numbers means joining counts into one bigger count. Line the digits up by place, start with the ones, and carry tens across when a column passes nine. That carry is just regrouping ten ones as one ten. You can check by adding in a new order, since order does not change the total. Quick recall of facts to 20 makes every bigger sum faster.
1.5: Subtract Whole Numbers (Part 1)
Subtracting whole numbers means finding what is left or how far apart two counts sit. Start with the ones column, and borrow a ten whenever the top digit is too small. Borrowing swaps one ten for ten ones, so the value never changes. Check with addition, because the missing part plus the taken part must rebuild the start.
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2 · Watch
به انگلیسیTake it off screen
Where it sits
Learn first
Optional connections
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 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.