7. 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 registered for the "Hua Cup Competition" is 1 less than 3 times the number of students who did not register. There are $\qquad$ students in this class.
Analysis: The number of female students is twice the number of male students, so the total number of students in the class is a multiple of 3, and the total number of students 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 plus 1 is a multiple of 4; Testing the total number of students in the class as 27.