Almost every percentage question is one of three jobs. The good news: the engine is the same each time — a percent is a fraction over 100, and "of" means multiply.
1 — a Percentage of an Amount
Turn the percent into a fraction or decimal, then multiply by the amount. 15% of 80: write 15% = 15/100 = 0.15, then 0.15 × 80 = 12. Done. (A handy mental trick: 10% is just the amount with the decimal slid one place left, so 10% of 80 is 8, and 5% is half of that — 4 — giving 12 again.)
2 — Increase or Decrease by a Percentage
First work out the change, then add it on (increase) or take it off (decrease). To grow 80 by 25%: the change is 0.25 × 80 = 20, so the new amount is 80 + 20 = 100. To shrink it by 25% instead, you'd do 80 − 20 = 60. Same first step, opposite second step.
3 — One Quantity as a Percentage of Another
This one runs backwards: you're handed two numbers and asked what percent one is of the other. Make the fraction part / whole, then multiply by 100. "12 is what percent of 80?" → 12/80 = 0.15, and 0.15 × 100 = 15%. The ×100 is what turns a plain fraction into a percentage.
Decimals Make It Smooth
Once you're comfortable, skip straight to the decimal: 37% is 0.37, so 37% of anything is just 0.37 × it. Most calculator work lives here. The fraction-over-100 version is just the same idea written long-hand.