I used string formating to write the data to the file!

I have improved on my gas mileage GUI program, by writing text to a file using string formatting. I was just writing the variable to the file, but would add extra stuff I didn't want to the file.  String formatting is a much better way to write data. It took me awhile to figure out this simple concept! Also, I added a button that will list the text file using a for loop. The for loop is a very nice way to list lines in a file.