Question

A bag contains 5 red marbles, 4 blue marbles, 3 green marbles, and no other marbles. If a marble is chosen at random from the bag, what is the probability that the marble chosen will not be red?

Answer 1

Answer:

A = choosing a red marble

B = choosing a green marble

Probability of A happening = P(A) = 5/(3+5+4) = 5/12

Probability of B happening = P(B) = 4/(3+5+4) = 4/12

Probability of either happening = P(A) + P(B) = 5/12 + 4/12 = 7/12

The key word in their question is red OR green and that is why we add the two probabilities. If they ask the probability of them both happening (if they said red AND green), you would multiply P(A)*P(B).

Answer 2

The answer to this question is $\frac{7}{12}$