Question

My car gets 13 miles per gallon in the city and 30 miles per gallon on the highway. Suppose I used 15.867 gallons of gas and got an average of 15 miles per gallon for the trip.

Answer 1

I assume you want to know how many miles in city and highway. Well, take a look at the data:

miles = gals * mi/gal so
total miles: 15.867*15 = 238

If there were c city miles, then the rest (238-x) were highway miles. So,

x/13 + (238-x)/30 = 15.867
x = 182

so,
182 city miles
56 hwy miles