Percent of means multiply by hundredths

X% of Y is (X/100) times Y — and "Y plus X%" is Y times (1 + X/100).

Why it works

Percent just means per hundred. The same algebra powers tips, discounts, VAT-style add-ons, and reverse "what percent is A of B".

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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