01 / MENTAL MATHS
Multiply by 11.
Keep the outside digits. Add them in the middle. Then discover what happens when you need to carry.
Try the exampleWATCH IT. TRY IT. UNDERSTAND IT.
Flip the percentage.
If 8% of 50 feels awkward, try 50% of 8. Swap the numbers, keep the answer, and see why it works.
The example is loaded. Try a different number whenever you’re ready.
YOUR INTERACTIVE EXAMPLE
Make it yours.
A LITTLE PRACTICE. A REAL “I GET IT”.
Your turn.
Can you call it?
Make a prediction before you reveal the answer. Use the method you just explored.
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Challenge 1 of 5
THE BIT THAT MAKES IT CLICK
Why does it work?
A percent means “divide by 100”. So a% of b = (a × b) ÷ 100. Swapping a and b gives (b × a) ÷ 100, the same product. This identity also works above 100%; it does not mean that a percentage increase and a discount are interchangeable.
Where this experiment applies
Try non-negative numbers up to 1,000, with at most two decimal places in each input. This experiment calculates a percentage of a number, not a percentage change.
THE RULE, IN ONE LINE
a% of b = b% of a
The explanation and the answer use the same numbers.KEEP THAT CURIOSITY GOING
One more “aha”?
03 / EVERYDAY MONEY
Two discounts. One surprise.
20% off, then another 20%, gives 36% off overall. Change the offers and watch what happens to the price.
Try the example