The math school offers three interest classes for students: calligraphy, art and chorus. There are 200 students in grade seven. ...

Let calligraphy class x and art class 2x. It is concluded that the chorus class is 200-3x ... because chorus+calligraphy accounts for 55% of the total number.

So [(200-3x)+x]/200=55%.

X has 45 students, including 45 in calligraphy class, 90 in art class and 65 in chorus class.