Math CentralQuandaries & Queries


Question from jonathan, a student:

Please explain how to solve this problem. I know you have to work backwards. If i have $125 to spend, how do I know the most I can spend before adding 6% tax and 9% shipping.(shipping is cost before tax) I can only figure this out by trial and error. I know there is a better way

Hi Jonathan,

Since the 6% tax and 9% shipping are calculated on your price you can add them. Thus your price plus 15% of your price must be $125. Suppose $P is your price then

$P + 15% of $P = $125


$P + 0.15 × $P = $125
1.15 × $P = $125

Now if divide both sides by 1.15 you get

$P = $125/1.15 = $108.70.

I hope this helps,

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