Xiaowen is one year old and Xiaoli is one to 27 years old. How old are they three years later?

Solution: because xiaowen's age is equal to a+3 years after three years. Li Li's age is equal to a-27 plus 3 and a-24 (years old), so the age difference between them after three years is a+3-(a-24)=a+3-a+24=27 (years old).

Through this question, we can also see that their age difference is constant. This year, the difference is that a-(a-27)=27 years old. Three years later, they are still 27 years old.