Mental multiplication and division
Use place value, known and derived facts to multiply and divide mentally, including multiplying by 0 and 1, dividing by 1, and multiplying together three numbers
What a learner can do afterwards
- Calculate 40 × 6 = 240 mentally using place value
- Explain why any number × 0 = 0 and any number × 1 = the number itself
- Multiply three numbers mentally by choosing a useful pair first (e.g. 2 × 7 × 5 = 2 × 5 × 7 = 70)
The lesson
You already know your times tables. Now you can use that knowledge to solve bigger problems in your head, without writing anything down.
To work out 40 × 6, first solve the easy fact: 4 × 6 = 24. Then remember that 40 is really 4 tens, so multiply your answer by 10. 24 × 10 = 240.
Two facts are so useful you can use them straight away, with no working out. Any number times 0 is 0, because you have zero groups of that number. Any number times 1 stays exactly the same, because one group of something is just that many.
Look for a pair that makes 10, 20, or 100. Multiply that pair first, then multiply by the number that is left.
Use facts you already know, apply the shortcut for × 0 and × 1, and reorder three numbers to find an easy pair first.
Watch it
Where it sits
This opens up
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.