My first instinct was also C but I think you can get B.
You have to develop the equation for the first 40 mile trip with the two different speeds.
Then for the second which will be T = 40/x
Since it is asking for a percentage you can further solve them by expressing in the percent form which should cancel x and then you substitute for y and getet the answer.
I haven't solved it completely yet.
I might be completely wrong but sitting at work and it is past 4pm and I am unable to think straight right now