If you want to stop depending on the calculator, you need a few reliable moves you can run without thinking. These four rules cover the vast majority of QR arithmetic. Master the first one — the percentage ladder — and a huge slice of the section becomes mental maths.
Rule 1: the percentage ladder
Almost any percentage can be built from four base moves: 10% (move the decimal point one place left), 5% (half of 10%), 1% (move the decimal two places left), and 25% (a quarter, or halve and halve again). Find the base values once, then add or subtract them like rungs on a ladder to reach the percentage you want. No keypad required.
The ladder in action
Say you need 35% of 240. Find 10% (24) and 25% (60), then add: 24 plus 60 is 84. Need 17% of 300? Take 10% (30), 5% (15) and 1% (3), then add 30 + 15 + 3 to get 48. Need 23% of 80? Start at 25% (20) and step down 2% (1.6), giving 18.4. You are never doing real multiplication — just halving, shifting decimals, and adding small whole numbers.
Why the ladder beats the calculator here
By the time you have selected the calculator, typed 240, pressed multiply, typed 0.35 and read the screen, the ladder has already given you 84 — and you cannot fat-finger a key you never pressed. The ladder also makes estimation natural: if the answer options are far apart, a quick 10% plus 25% is often enough to pick the right one without finishing the sum.
Rule 2: round, then correct
Awkward numbers get friendly when you round them. To work with 19.6, treat it as 20 and remember you have added 0.4. To handle a 2.5% surcharge on a bill, take 1% three times for 3%, then ease back — or simply find 2% and 0.5% and add. Rounding to a clean number, doing the easy sum, then nudging the result back is faster and far less error-prone than typing long decimals.
Rule 3: simplify with fractions
Fractions are often quicker than percentages and decimals. A quarter is 25%, a fifth is 20%, an eighth is 12.5%, three-quarters is 75%. To find seven-eighths of a quantity, it is usually faster to find one-eighth and take it away from the whole than to multiply by 0.875. When a percentage maps cleanly onto a simple fraction, switch — your head handles dividing by 4 far better than multiplying by 0.25.
Rule 4: cancel before you calculate
When a question becomes a fraction or a ratio, shrink the numbers before you do anything. 150 out of 600 is just one-quarter once you cancel. A ratio of 40 to 100 is 2 to 5. Dividing 4,200 by 0.75 is the same as multiplying by four-thirds, which is far kinder by hand. Cleaning up the numbers first means the actual arithmetic is something you can finish mentally in a couple of seconds.