Percentages in your head, no calculator.
Smart Mathematician Fractions & Percentages Maths Games for Teenagers Maths Games for Adults
Every round is a percentage you should be able to do mentally by decomposing it. 15% is 10% plus half of 10%. 20% is a fifth. 5% is half of 10%. Once you see percentages as combinations of easy pieces, the calculator becomes optional.
The discount and tip variants apply exactly the same arithmetic to situations where you actually need it, which is the point — a skill you only use in a maths game is not really a skill.
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Find 10% first, by moving the decimal point one place left. Almost every other percentage is built from that: 5% is half of it, 20% is double, 15% is one and a half times.
Yes, always, and it is the most underused shortcut in mental arithmetic. 4% of 75 looks unpleasant; 75% of 4 is three.
They use the 10, 15 and 20 percent bands that are common in practice, so the practice transfers directly.