>>11492734>>11492737I think there's an even simpler explanation.
If you take out the condition that a gold ball is picked, there are six possiblities:
>box a, ball 1>box a, ball 2>box b, ball 1>box b, ball 2>box c, ball 1>box c, ball 2Knowing that we got a gold ball eliminates the bottom three possibilities, leaving us with three. Two of those three are box a, so the chance is 2/3rds.
