Mental calculation strategies
Perform mental calculations including with mixed operations and large numbers, using strategies such as partitioning, compensation, and derived facts
What a learner can do afterwards
- Mentally compute 45 × 8 by partitioning into 40 × 8 + 5 × 8
- Use known facts to derive 6.5 × 4 mentally
- Solve a multi-operation mental calculation such as 250 × 3 + 500 and explain the strategy used
1 · Read
At the till with no calculator, you still need to know what six packs at 52p each will cost before you pay. Mental maths asks you to do that job in your head, by breaking one hard problem into smaller, friendlier ones.
To work out 52 × 6 in your head: split 52 into 50 + 2. Work out 50 × 6 = 300 and 2 × 6 = 12. Add them together: 300 + 12 = 312. When one problem has both multiply and add or take away, do the multiply part first.
Derived facts save you work. If you already know 6 × 4 = 24, you don't need to start over for 6.5 × 4. Just add half of 4, which is 2, so 6.5 × 4 = 26.
When a problem has more than one step, like 250 × 3 + 500, do the multiplication first, then the addition. Say each step out loud so you don't lose track: 750, then 750 + 500 = 1250.
Split a hard problem into easy pieces, use facts you already know, and take one step at a time.
The rewritten reading was written for this dataset.
2 · Watch
Take it off screen
Where it sits
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.