[ Dirichlet's Principle (continued).]

In a class of 25 students, it is known that any two girls in the class have a different number of boy friends from this class. What is the maximum number of girls that can be in this class?

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If there are 13 girls in the class, the number of their boy friends from this class can be any integer from 0 to 12 (13 different options), which meets the condition. If there are more than 13 girls (at least 14), then the number of boys in the class will not exceed 11, which means the number of different options for the number of boy friends will not exceed 12 (from 0 to 11). Therefore, at least two girls will have the same number of boy friends, which contradicts the condition.

## Answer

13.