Scale a value proportionally when you know the ratio. Useful for recipes (double an ingredient), maps (convert distances), currency (apply an exchange rate), or any situation where two things grow at the same rate.
A1 / B1 = A2 / B2 → B2 = ( B1 × A2 ) / A1
X% of Y — find a portion of a number (tax, tip, commission). What % — figure out how big a slice something is (score out of total, share of a budget). % change — measure growth or reduction between two values (price increase, weight loss, traffic change).
Discount — use when a price is being reduced: sales, coupons, negotiations. Gross margin — use when setting a sell price from a known cost. Unlike markup (profit over cost), margin tells you what percentage of revenue is profit, which is how most businesses report it.
Discount
Gross margin (profit / sell price)