Question

Margo has 3 blue marbles, 4 green marbles,and 5 red marbles in a bag. She randomly selects one marble. What is the probability that the marble is green or red?

Answer 1

Prob(green) =  number of green marbles / total number of marbles =  4/12 = 1/3

Prob(red) =  5/12

Here we add probabilities  so the answer is 1/3 + 5/12 =  9/12 = 3/4

answer is 3/4  or 75%

Answer 2

First you must add up all the marbles in the bag.

3 blue marbles
4 green marbles
5 red marbles

3 + 4 + 5 = 12

You are looking for the probability of picking up either a green or red marble.

So not counting blue marbles, there are 9 green and red.

9/12 = 0.75

The probability of picking up either a green or red marble is 75%.