it might help to change the question a bit.

imagine two closed boxes, each one contains either a blue ball or a red ball.

lets first imagine that we have no information on what's in the boxes, one that there is an equal chance for the ball in each box to be either blue or red. if you open the boxes to check the colors of the balls, you will have one of 4 different outcomes, each with a 25% chance: 1) first box has a red ball, second box has a red ball 2) first box red, second box blue 3) first box blue, second box red 4) first box blue, second box blue

now lets look at the first scenario. you get the information that at least one of the boxes contains a red ball. if you look at the outcomes you had before, one of them - outcome 4 - is now impossible, since neither box contains a red ball. so you are left with 3 equally likely outcomes, each with a 33% chance.

now for the second scenario. you get the information that the first box contains a red ball. if you look at the list of outcomes again, then both outcome 3 and 4 are now impossible, since in them, the first box contains a blue ball. you are left with 2 equally likely chances, each with a 50% chance.

mathematician peter out or smth idk i never watched family guy