Marbles With An Equal Chance

Discrete Mathematics Level 3

A box contains two white marbles and two black marbles. I pick a marble at random and set it aside. Then, I pick a second marble and notice that it is black.

Is it more likely that the first marble was white or black?

Excel in math and science

Master concepts by solving fun, challenging problems.

It's hard to learn from lectures and videos

Learn more effectively through short, conceptual quizzes.

Our wiki is made for math and science

Master advanced concepts through explanations, examples, and problems from the community.

Used and loved by 4 million people

Learn from a vibrant community of students and enthusiasts, including olympiad champions, researchers, and professionals.