Let X = total students in musical
Then (1/3) X = number of seniors
1 + 6 + 11 + (1/3)X = X
18 + (1/3)X = X
18 = (2/3)X
(3/2)(18) = X
27 = X
27 students in musical of which one third are seniors,
there are 9 seniors in musical
hope this helped!