GMAT Practice Question: Ratios

The ratio of the number of students, teachers, and administrators at Charter School is 5 to 2 to 1.  If the ratio of the students to administrators is doubled and the ratio of the teachers to administrators is halved, then how many students are at the school if there are four administrators left after the changes?

A) 10

B) 20

C) 30

D) 40

E) 50