Percentages
of, off, adding and subtracting percent, and percent questions.
Percentages read the way you say them. "20% of X" takes the percentage, "X + 20%" adds it on (how tips and taxes work), and "20% off X" takes it away.
Percent questions
You can ask in plain words, in both directions. "A is what % of B" gives the share; "A is P% of what" finds the original. For the change between two numbers, write "percent change from A to B" (or the shorter "A to B is what %").
Percents as numbers
Percents of percents just add: 10% + 20% is 30%. And any number or fraction converts with "as %".
Bare percents in variables
A line like tax = 21% stores the rate and remembers that it is a percent. Multiply by it to get the fraction, and add or subtract it to apply it, exactly like the literal: 2500 - tax is 1,975, the same as 2500 - 21%.
The exact rules, for power users
Adding or subtracting a percent applies it to everything on its left, and repeated percents compound left to right: 100 + 10% + 5% is 115.50, not 115. By contrast, "% of" binds only to the term right after "of": 20% of 200 + 30 means (20% of 200) + 30, which is 70. With * and /, a percent is just its fraction: 200 * 10% is 20. One special case: sitting between two plain numbers, % is the remainder (modulo) instead, so 10 % 3 is 1. Use parentheses when in doubt.