Sister Fangfang 12 only had three birthdays. What year could she have been born?

In 2004. Sister Fangfang 12 only had three birthdays. Sister Fangfang should have been born in 2004, and 12 divided by 3 equals 4, which means Sister Fangfang only has a birthday every four years, which means Sister Fangfang was born on February 29th of a leap year. 2004 divided by 4 equals 50 1, 2004 is a leap year, 1990 and 2006 are normal years. Then Fangfang's sister should have been born in 2004.