Question

A drawer contains 10 blue pens, 10 black pens, and 10 red pens. Without looking. Mr. Lopez is going to take one pen from the drawer, use it, and then put it back into the drawer. Then he is going to take another pen from the drawer to use. What is the probability of Mr. Lopez taking a red pen first and then taking a blue pen?

Answer 1

1/9

Explanation:

There are 30 pens in total, 1/3 of the pens are each the following colours: blue; black; red.

This will result in a one in three chance of picking up a red pen. As Mr.Lopez puts the pen back, the fractions stay the same as it's the same possibility both times.

As it's again a one in three chance, we just have to multiply it for the possibility of him picking a blue pen, which is 1/3.

1/3 * 1/3 = 1/9