Question 42427
Ok to solve this, we first have to set up our variables.

Let n= The amount of oil used in September
Let n+12= Amount of Oil used in October
Let 2n= Amount of oil used in November

So now we have to add everything up and have it total 132 gallons of gas, because that's how much they used.

n+n+12+2n= 132

Now we combine like terms, and solve for n.

4n+12=132
  -12=-12
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4n=120
/4= /4

n=30

Ok so we now know that n=30, now we can plug thirty for n and figure out how much oil they used for each month.

20= September
32= October
40= November