A probability is the likelihood of a successful event occurring divided by the total number of events possible. In this case, a successful event is selecting either a red or a yellow marble and the total number of events possible is the total number of marbles. Combine the number of red and yellow marbles: 4 + 3 = 7, and divide this by the total number of marbles: 5 + 4 + 3 = 12. The probability is .