There are 270 students at Colfax Middle School, where the ratio of boys to girls is 5: 4. There are 180 students at Winthrop Middle School, where the ratio of boys to girls is 4: 5. The two schools hold a dance and all students from both schools attend. What fraction of the students at the dance are girls?
Answer Choices
A. \dfrac{7}{18}
B. \dfrac{7}{15}
C. \dfrac{22}{45}
D. \dfrac{1}{2}
E. \dfrac{23}{45}