Why does my child multiply by 0.2 when something loses 20%?
Your child multiplies by the percentage instead of by what's left, such as losing 20% a year as × 0.2 instead of × 0.8.
Why children do it
The 20% is the number in the question, so it's the one children multiply by. But that's the part that's lost: what's left is 80%, so the value is multiplied by 0.8 each year. In the same way, growing by 5% is multiplying by 1.05.
How to help at home
Ask them what's left after losing 20%: 80%. That's the number to multiply by.
The right way
A boat bought for $10000 loses 20% of its value each year. What is it worth after 2 years?
- 10000 × 0.8 × 0.8Losing 20% leaves 80%: multiply by 0.8 each year.
- 6400It's worth $6400.
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