Some months have 31 days and some have 30. How many have 28?

How to think about it

1. The instinctive reply is one month, February, because it is the only month with exactly 28 days in a common year.
2. Reread the question. It asks how many months have 28 days, not how many have only 28.
3. A month with 31 days certainly has 28 of them, and so does a month with 30.
4. Every month of the year has at least 28 days, so the answer is all twelve.

Why “All of them” is the answer

Each of the twelve months contains a 28th day, so every month “has 28 days” even though most of them have more. The riddle sets a trap by mentioning 31 and 30 first, which makes you treat 28 as a category instead of a minimum. February is special only because it has nothing beyond the 28th in a common year.

Common wrong answers

• One. February is the only month with exactly 28 days, but the question does not say “exactly”.
• None. In a leap year February has 29, but it still has a 28th day, and so does every other month.