Practice!

Sample Problem

Mr. Popular received 366 greeting cards in 1987. Are there any cards sent on the same day? (1987 is not a leap year.)

Solution

The year 1987 – or any non-leap year, for that matter – has 365 days. Imagine putting 366 pigeons in 365 holes. We can see that we must put more than one pigeon into at least one pigeonhole, so there must be a day on which more than one card is sent.