Stage 8 · Fractions & decimals
Percentages in your head
Find 10% first, then double or halve.
How to do it
Method
Find 10% first, then double or halve
20% of 80: 10% is 8, 8 × 2 = 16
Another way
As a fraction
80 × 20100 = 16
In full
Find 10% first (÷10, the point moves one place left); 5% is half of 10%, 20% is double 10%. 50% is half the whole, 25% a quarter of the whole, 75% is 50% + 25%. 25% of 80 → 80 ÷ 4 = 20. Or turn the percentage into a fraction and multiply: 25% of 80 = 80 × 25100 = 20 — the same answer.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
10% of 40
Show the answer and working
Answer: 10% of 40 = 4
10% of 40: 10% is 40 ÷ 10 = 4
Example 2
10% of 300
Show the answer and working
Answer: 10% of 300 = 30
10% of 300: 10% is 300 ÷ 10 = 30
Example 3
5% of 400
Show the answer and working
Answer: 5% of 400 = 20
5% of 400: find 10% first, 400 ÷ 10 = 40, 5% is half of 10%, 40 ÷ 2 = 20
Example 4
5% of 200
Show the answer and working
Answer: 5% of 200 = 10
5% of 200: find 10% first, 200 ÷ 10 = 20, 5% is half of 10%, 20 ÷ 2 = 10
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