Actually oak, that makes sense. So he has:
Staying in A - 50% * 50% = 25% chance of pardon
Switching to B- 50% * 50% = 25% chance of pardon
Switching to B(alt. Scenario)- 50% * 100% = 50% chance of pardon
So the way I see it, switching to B actually gives you a 75% chance of pardon, under that assumption about the guard telling him. I originally assumed the guard would tell him. Anyway I was wrong, need to admit it. Better to switch to B. Anyway, makes my head spin. Good stuff