James car averages 13.6 miles per gallon in city driving and 25.9 miles per gallon in highway driving. If he drove a total of 515.8 miles on 31 gallons of gas, then how many of the gallons were used for city driving?

• If x gallons are used on highway, then
  
  25.9x +13.6*(31-x)= 515.8
  
  solving it, we get x= 7.66 gallons.
  gas used in city = 31-7.66 = 23.34 gallons
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