[yorwba]

I interpreted it as the bus starting at 9am and stopping at 9pm, making for 12 trips of one hour each. So the off-by-one error would be in the code generating the possible trips.

In fact, the code as shown only produces 48 trips, instead of the 52 given. I had to actually run it to make sure the Python interpreter in my had wasn't buggy. No idea what code they actually ran to get that result.