Why does my child think a price of $110 after a 10% rise was $99 before?
Your child undoes a 10% increase by taking 10% off the new price, such as $110 back to $99 instead of $100.
Why children do it
Taking 10% off seems to undo adding 10%. But the two 10%s are of different amounts: the rise was 10% of the old price, and taking 10% off the new price takes off more. The old price is the new one divided by 1.1, which is $100.
How to help at home
Check by going forwards: does 10% more than the answer give the price in the question?
The right way
After a 25% increase, a price is $100. What was the price before the increase?
- 100 ÷ 1.25After going up 25%, the price is 125% of the old one, so divide by 1.25.
- $80Check: 25% of $80 is $20, and $80 + $20 is $100.
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