An old man had been browsing for some time. He finally walked up to the girl behind the counter and handed her a book. She looked at the tag on the inside cover and said, "That's £4.50 please." The man handed her the money and promptly walked away without the book. The girl saw him leave empty-handed but she didn't call out to him in any way. Why?
The old man was paying a fine for an overdue book in a library.
What is a prime number?