Question 710509
Your guess (not a very good mathematical term) is almost correct except for the X on the right hand side of
(1) 35 + 0.10*X > 45 + X
It should be
(2) 35 + 0.10*X > 45, without the X because you get unlimited milage for $45. That is, in math terms it is independent of mileage X.
Solve (2) gives us
(3) 0.10*X > 45-35 or
(4) X/10 > 10 or
(5) X > 100
Answer: Plan B is better when you are going to travel more than 100 miles a day.