Mental maths strategies · Grade 5
In Grade 5 you learn: multiplying and dividing by 10, 100 and 1,000.
Mental maths is about choosing a clever route through a calculation instead of always using the same written method. Rounding and adjusting, partitioning, doubling and halving, and using facts you already know turn hard-looking sums into easy ones.
Key facts
- Round and adjust: + 99 is + 100 − 1; − 49 is − 50 + 1.
- Partition: 6 × 23 = 6 × 20 + 6 × 3.
- × 5 is × 10 then halve; × 25 is × 100 then ÷ 4.
- For numbers close together, count up to find the difference: 1,003 − 996 = 7.
Worked examples
Example 1
347 + 199
Answer: 546
Example 2
48 × 5
Answer: 240
Example 3
7 × 49
Answer: 343
Example 4
4 notebooks cost R2.99 each. What is the total?
Answer: R11.96