Ok, so
Remember the following equation:

Speed = (Distance) / (Time)
As we know, Bus A travels 600 miles from Portland to San Franciso at an average rate of 50 mph. So the time it takes it to arrive will be:

Bus B makes the same trip traveling at an average rate of 60 mph and makes a two hour stop along the way.
So,

As it makes a two hour stop along the way, the total time to arrive of the bus B, will be 12h too.
So, both buses arrive at the same time.