Oops! Let's check our math! Each part of the gauge does show 5 gallons, but we need to figure miles per gallon. For this problem, you need to divide, not subtract.
Look at it this way: Maggie knew she could divide the number of miles by the number of gallons to get "miles per gallon." The word "per" is another way of saying "divided by," so Maggie divided 575 miles by 50 gallons to get 11.5 miles per gallon.