02 / PERCENTAGES
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 exampleWATCH IT. TRY IT. UNDERSTAND IT.
Multiply by 11.
Keep the outside digits. Add them in the middle. Then discover what happens when you need to carry.
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?
Multiplying by 11 means multiplying by 10 and adding the original number. If n = 10a + b, then 11n = 100a + 10(a + b) + b. A middle sum above 9 carries into the place to its left. A carry can also make the result four digits, as with 99 × 11.
Where this experiment applies
This shortcut is presented for whole numbers from 10 to 99, in base ten. It is not the complete rule for longer numbers.
THE RULE, IN ONE LINE
n × 11 = n × 10 + n
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