Start from zero — the basic math you need before everything else
Percent means "per hundred", or out of 100. So 25% is 25 out of 100, which is the fraction 25/100 = 1/4, or the decimal 0.25. Percent, fraction and decimal are three ways of saying the same thing.
To find a percent of a number, turn the percent into a decimal and multiply. A few are worth memorising: 50% is a half, 25% a quarter, 10% a tenth. A 20%-off price means you keep 80% of it, so a £50 item costs 0.8 × 50 = £40.
You read percentages all day. A phone battery at 50% is half full; at 25% it's a quarter. A test marked 18 out of 20 is 18/20 = 90%. 'Per cent' just means 'out of a hundred', so any score becomes a percentage by scaling it to 100.
▶ Percentages