The one idea: multiply by a factor
A standard method from Percentages: the steps.
Steps
| Change | Multiply by |
|---|---|
| increase by r% | (1 + r%) |
| decrease by r% | (1 − r%) |
| two changes in a row | multiply both factors |
Why: "increase by 20%" means "keep 100% and add 20%", which is 120% = 1.2.
For the whole topic, read Percentages: key points, common mistakes and how the DSE tests it.