A 25% off sign is an instruction with a shortcut in it: $S = L\,(1 - d)$ — read aloud: S equals L times one-minus-d. Meet the three letters first: $L$ is the list price, the dollars on the original tag; $d$ is the discount, the sign's percent as a decimal (25% becomes 0.25); and $S$ is the sale price you actually hand over — the one you are solving for here. So $(1 - d)$ is the fraction you still pay — 75 cents of every sticker dollar. One multiplication, done. The percent NUMBER never gets subtracted as dollars: 25% off an $80 jacket is $20 off, not $25 off.

Running it backwards is where sales quizzes love to live: the tag shows the SALE price and asks for the original. The sale price is $(1 - d)$ of the original, so you divide by $(1 - d)$ — never add the percent back on. Adding it back measures the discount against the smaller number, and the store took it off the bigger one. Undo a multiply with a divide, every time.