>>53756126>>53756144I will admit I did not realize you were approaching it from this angle.
but even still, since you have been guaranteed to pick one of the boxes that has a gold ball, and you are also guaranteed to have picked the gold ball first, it means that there are only two remaining possibilities of what the second ball in the same box is.
there are three balls to choose from, yes, but you also have to keep in mind that the hypothetical specifies that the box is picked before the balls, meaning that you aren't choosing from three gold balls, you're choosing from two boxes that happen to have gold balls in them.
having the possibility of picking up the silver ball first would have absolutely muddied the probability, but like with the boxes, you have been explicitly guaranteed that it was not picked up first, so there are only two possible colors of what the remaining ball could be, and both have an equal chance of having been picked due to there only being two boxes to choose between
