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.
2 · Watch
Take it off screen
Where it sits
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.