Four percentage questions cover almost everything people actually need: what is X percent of a number, what percentage is one number of another, what is the percentage change between two numbers, and what number does a known percentage come from. All four are below, each showing the formula it used so you can check the working or reuse it.
X% of Y converts the percentage to a decimal and multiplies. This is the one you use for tips, tax and discounts.
X is what percent of Y divides and multiplies by 100. Use it for scores, market share and any "how much of the total" question.
Percentage change divides the difference by the starting value. Which number you start from matters: 50 to 100 is a 100% increase, but 100 back to 50 is a 50% decrease, even though the gap is the same.
Reverse percentage recovers the original from a part and its percentage. If 30 is 15% of something, that something is 30 / 0.15 = 200. This is how you strip a known markup out of a final figure.
Percentages do not commute across different bases. A 20% cut followed by a 20% rise does not return you to where you started - you land at 96% of the original, because the rise is calculated on the smaller number. The same asymmetry is why a stock that falls 50% has to double just to break even.
Divide the percentage by 100 and multiply by the number. 15% of 240 is 0.15 x 240 = 36.
Percentage change is relative to the starting value: going from 4% to 5% is a 25% increase. Percentage points are the absolute gap: the same move is a rise of 1 percentage point. Interest rates and poll results are usually quoted in percentage points.
Each percentage is taken from a different base. 100 plus 10% is 110, and 10% of 110 is 11, so you land on 99. To reverse a 10% increase, divide by 1.1 rather than subtracting 10%.
Steuervergleich Kanton Zürich — Gemeindesteuern vergleichen (Deutsch)