Christmas eve!

Discrete Mathematics Level 5

On the Christmas Eve, Santa was in a hurry to distribute gifts, so he asked the most innocent child in the country to help him in distributing 10 gifts to 10 different houses, where each house receives a gift.

To House #1, the boy gave the gift of House #2. At the end of the day, Santa informed the boy that he had not distributed a single gift to the correct place.

In how many ways can this happen?

Ignore the order in which the gifts were distributed. Consider just which gifts the houses received.

This is part of my set "Fun is a Student's Right."
You would also like "Let's play with polygons & circles."

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, interactive explorations.

Used and loved by over 7 million people

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