Bit of a hint on the Arcade Mode:
For the single races, you can just go through on the "hard" setting. If you win the hard race, it'll give you a win for both the easy and the medium at the same time. If you sit down and run through all of these, you'll probably end up at over 75%, with a nice new car.
And...the time trials are fairly hard. They're pretty much the S licence all over again, but different tracks. You may want to leave these off till you've got the S and a good chunk of Professional done.