Multiply by 11.
Keep the outside digits. Add them in the middle. Then discover what happens when you need to carry.
Try the exampleDouble discount calculator
20% off, then another 20%, gives 36% off overall. Change the offers and watch what happens to the price.
Your numbers
The example is loaded. Try a different number whenever you’re ready.
Make your prediction.
Make a prediction before you reveal the answer. Use the method you just explored.
0 challenges solved this session
Challenge 1 of 5
Why does it work?
The first discount leaves a fraction (1 − a/100) of the original price. The second applies to that reduced amount, leaving another fraction (1 − b/100). Multiply those fractions together. The combined percentage discount is a + b − ab/100, not simply a + b.
Where this experiment applies
Percentage reductions only: 0–100% each; price $0–$100,000. The model applies both factors before rounding the final amount to the nearest cent. A checkout that rounds at each stage can differ by a penny. Fees, tax changes and excluded items are outside this example.
THE RULE, IN ONE LINE
Keep (1 − a/100) × (1 − b/100)
The explanation and the answer use the same numbers.One more “aha”?
Flip the percentage.
If 8% of 50 feels awkward, try 50% of 8. Swap the numbers, keep the answer, and see why it works.
Try the example