# Probably a really simple math question.
**URL:** https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827
**Category:** Factual Questions
**Created:** [November 26, 2013, 4:05pm UTC](https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827 "2013-11-26T16:05:09Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Kimballkid](https://boards.straightdope.com/u/Kimballkid)
#### Post date: [November 26, 2013, 4:05pm UTC](https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827/1 "2013-11-26T16:05:09Z")
And I’m either brain farting or way over thinking it.
Some background: Around here 10% ethanol gas is anywhere from 20 to 40 cents cheaper than regular. What I wish to do is figure out which is more cost effective to use in terms of mileage and price.
Here are the numbers from one of my latest receipts:
Cost per gallon (CPG): $2.999
Gallons: 6.700
Total Price (TP): $20.09
Total Miles Driven (TMD): 282.0
Miles Per Gallon (MPG): 42.1
Now my thought is to take CPG/MPG and use that number. Can it really be that simple or is it more complicated? Should I be using TP/TMD instead? I’m sure when someone answers, it will be a big ol’ :smack: . Thanks.
Also, I really don’t want nor do I care to debate the pros and cons of ethanol. If you wish to do that, take it to one of the umpteen other threads about that.
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### Author:  [@bup](https://boards.straightdope.com/u/bup)
#### Post date: [November 26, 2013, 4:14pm UTC](https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827/2 "2013-11-26T16:14:54Z")
What you want is Cost per mile.
Cost per gallon divided by miles per gallon:
C/G
* * *
M/G
is the same as Cost per gallon _times_ gallons per mile (it’s just inverting the fraction):
C/G \* G/M
In that case, gallons cancel out. So yes, C/G divided by M/G leaves you with cost per mile.
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### Author:  [@Baracus](https://boards.straightdope.com/u/Baracus)
#### Post date: [November 26, 2013, 4:17pm UTC](https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827/3 "2013-11-26T16:17:37Z")
Choose whichever moves you because they both give you the same answer.
The cost per gallon is the total price divided by the gallons. If you substituted that into the first expression you would have TP/(MPG_G). MPG_G equals TMD, hence the expressions both give the same result.
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### Author:  [@Kimballkid](https://boards.straightdope.com/u/Kimballkid)
#### Post date: [November 26, 2013, 4:20pm UTC](https://boards.straightdope.com/t/probably-a-really-simple-math-question/674827/4 "2013-11-26T16:20:17Z")
Cool. Thanks.
I don’t know why I thought it had to be more complicated than that.