IGCSE Maths / Ratios and percentages
Writing one number as a percentage of another revision notes
Edexcel International GCSE Maths (4MA1), chapter 2: Ratios and percentages, specification point 1.6b-f. Part of the full course.
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To write one number as a percentage of another, put the part over the whole, then multiply by 100. This lets you compare amounts even when their totals are different.