a. 260 + 0.29x = 419.5
b. 550 miles 
How to: 
•260 is a number that you subtract from the total
•0.26 is what your trying to find how many of (miles in this example) so you put a variable (the letter) after it
•419.5 is the total
260 + 0.29x = 419.5     Subtract whole #
-260             - 260
                     —————-
            0.29x = 159.5.           Divide #x
         ______.  ______
             0.29.     0.29
X= 550