First let's assume that all the games are uniformly distributed i.e. any game is equaly likely to be found in a random box of cereal. That means that probability of getting any of the five games in a randomly selected box is `p=1/5` (because there are 5 games).
Since probability that a game is in a random box is `p=1/5` then probability that game is not in a random box is `q=1-1/5=4/5`. Now since our choosing of boxes is mutally independent probability that a game is not found in any of 12 boxes is `(4/5)^12 approx 0.0687` which means that probability of getting all 5 games from 12 boxes is approximately `1-0.0687=0,9313`.