Percentage calculator

Each question is one formula. p% of x is x × p ÷ 100. a as a percent of b is a ÷ b × 100. The change from a to b is (b − a) ÷ a × 100, printed with its sign and the difference itself. x increased by p% is x × (100 + p) ÷ 100, and decreased, x × (100 − p) ÷ 100. The value before a change is the last one run backwards: y × 100 ÷ (100 + p) for an increase, y × 100 ÷ (100 − p) for a decrease — which is why taking 15% off 92 does not give back 80.

The arithmetic is exact: every number is read as a decimal fraction held in whole-number BigInts, not as a floating-point number, so 0.1 + 0.2 stays 0.3 and a price keeps its cents. An answer that does not end within 10 decimal places — 1 as a percent of 3 — is rounded half up to 10 places and marked with ≈; Copy copies the printed digits. A percentage change from 0 has no value, and neither does a percent of 0 or the value before a 100% decrease; each says why instead of printing Infinity.

A number can have a leading sign, a decimal point or a decimal comma, spaces or apostrophes between digit groups, and a trailing %. When a number holds both a point and a comma, the last one is the decimal mark; a lone point is always a decimal point, and two or more points or commas are digit groups. A lone comma between one to three digits and exactly three more, as in 1,500, could mean fifteen hundred or one and a half, so it is refused with that question rather than guessed. A number takes up to 40 characters. Nothing is uploaded: there is no server in this file.

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