Answer:
It is 28 miles. It is much more simple than you are making it out to be in your head. Draw out the picture with 7 marks on a line. The first 3 marks were traveled Friday. After Friday, there are 4 marks left. On Saturday, 2 of those 4 remaining marks are traveled. On Sunday, the last two marks are traveled. The last two marks are 8 miles together, meaning that each mark is 4 miles a piece. Therefore, 7 times 8 is 28 miles.
Explanation: