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The old man is already sixty years old. But he has only fifteen birthdays. How is it? When is his birthday?
Keys: His birthday is on February 29th. It's a leap(闰年) year every four years. So he has only fifteen birthdays.
1. The old man is __________ years old.
2. He has only __________ birthdays.
3. His birthday is on __________.
4. A leap year happens(发生) every __________ years.
