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Question
Dear Mr. Hemmer,

My problem is in determining (roughly) the value of an item in 1980 based on today's price and Consumer Price Index.  I don't have the skills to subtract percentages over 100%.  
Given:  
Item is priced at $74 today.
CPI this month 230
CPI in 1980: 90
CPI difference: 140

Thank you.

Sincerely,

John Ganahl

Answer
Just solve 230/90=74/x for x by cross multiplying. You get 230x=90*74 so x=28.96. So assuming those CPI numbers are correct, $74 today was worth $28.86 in 1980.

Dave

Note the difference is irrelevant, it's the ratio that matters. Obviously if you gave me numbers of 30 and 170 instead of 90 and 230 the difference would be the same but the deflator is much different.

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David Hemmer

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I can answer almost any question from undergraduate mathematics courses.

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Mathematics professor.

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Ph.D. University of Chicago

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