A marble is selected at random from a jar containing 20 red​ marbles, 70 yellow​ marbles, and 10 green marbles. Find the theoretical probability that it is either red or green.

Answer 1

Because this asks for the probability or red OR green, you add the individual probabilities together. The probability of getting a red marble is 0.20 (20/100) and the probability of getting green is 0.10 (10/100) Therefore, when you add the two probabilities together, you get 0.30, or 30%.

Answer 2

The probability that a single marble drawn will be red is 20/100. The probability that a single marble drawn will be green is 10/100. So the probability that a single marble drawn will be either red or green is 20/100 + 10/100 = 30/100 = 3/10