One marble is to be selected from a box containing red, white, blue, yellow, and green marbles.

Question

One marble is to be selected from a box containing red, white, blue, yellow, and green marbles. If the probability that the selected marble will be red is 2/7 and the probability that it will be white is 3/7, what is the probability that it will be blue, yellow, or green?

Answer 1

Probability always adds to 1.0 (in other words, the probability is certain that when you reach in the bag and pull out a marble, the color will be either red, white, blue, yellow, or green). This is sort of obvious because that is all the color choices there are in the bag. But it's an important concept. Since you have a total of 1.0 probability and the probability of red is 2/7 and the probability of white is 3/7 that adds to 5/7. That means the probability of either a blue, yellow, or green MUST be the remaining 2/7 (because 5/7 + 2/7 = 1.0)