A bag has 7 red marbles, 8 blue marbles, and 5 purple marbles. Rachel draws a marble from the bag without looking.

What is the probability that Rachel draws a blue marble?

First you would add all the marbles together because you need to know how many are in the bag in total.

7 + 8 + 5 = 20

Next you simply put the number for blue over the total

8 / 20

And there's your answer.