Working Backward: The Original Price

Finn reads this

Sometimes you know the sale price and the discount, and the original price is the missing piece. The question that keeps you safe is: percent of what?

The shirt

Finn reads this

A shirt is 40 percent off and the sale price is $12. The $12 is not 100 percent of anything useful. It is the 60 percent that is left after 40 percent came off. So 60 percent is $12, 10 percent is $2, and the original price, 100 percent, is $20. The discount was $8.

The mistake to not make

Elle reads this

Adding 40 percent of $12 back on gives $12 plus $4.80, which is $16.80. That is wrong. The 40 percent was taken from the original price, $20, not from the sale price, $12. Forty percent of $20 is $8; forty percent of $12 is only $4.80. A percent is always a percent of a specific whole.

ElleMath-speak

A percent is always a percent of a specific whole.

FinnPlain English

Before you multiply, ask: percent of what?

Let me try a few

NC standards: NC.7.RP.3.

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