I have been 'tutoring' a grade school student as a favor to a family friend. This child has been struggling in the math's and science's. Years ago I tended to do well in the math's and science's, and so I agreed to try and help my godchild with her homework. Fast forward to today. I recently came across a seemingly trivial problem that I could not explain to her. I was able to come up with the correct answer based on intuition alone, but I couldn't explain to her the specific steps to solve this problem.

This single test-prep question has been bugging me since then. Luckily she was able to follow my reasoning and get this section of the test correct. I would just like to be able to explain WHY this question works out. I will type the question in a very similar format, but using rounded numbers. I'm not sure what my issue is, but for some reason this question is causing me a major brain-fart. Hopefully yall can lend me a hand.

"You are planning on taking a road trip from Boston, MA to Washington, DC. This trip is roughly 700 miles. You are driving an average vehicle that gets 20mpg. Assume gas costs an average of $3.50/gal. Using the numbers given, what is the total cost to take this trip?"