7. (10 points) The number of students in a class is greater than 20 and less than 30, and the number of female students is twice the number of male students. The number of students who signed up for the "Hua Cup Competition" is 1 less than 3 times the number of students who did not sign up. The class has $\qquad$ students.
【Analysis】The number of female students is twice that of male students, so the total number of students in the class is a multiple of 3, which can only be 21, 24, 27; The number of students in the class who have registered for the "Hua Cup Competition" is 1 less than 3 times the number of students who have not registered, so the total number of students in the class plus 1 person is a multiple of 4; The total number of students in the class is verified to be 27.

【Solution】Solution: According to the analysis, the total number of students in the class is a multiple of 3, which can only be 21, 24, 27; The number of students in the class who have registered for the "Hua Cup Competition" is 1 less than 3 times the number of students who have not registered, so the total number of students in the class plus 1 person is a multiple of 4; The total number of students in the class is verified to be 27.

Therefore, the answer is: 27.