In a school with 620 students, the average age of the boys is 12 years and that of the girls is 11 years. If the average age of the school is 11 years 9 months, then the number of girls in the school is
Pick.
A) 150  B) 200  C) 155  D) 350  E) None
Sol.   Let the number of grils be x.   Then, number of boys = (620 - x).   Then, (11 3/4 × 620)   ⇔ 11x + 12(620 - x) ⇔ x = 7440 - 7285 ⇔ 155.   Answer 155
<result>C</result>
In a school with 620 students, the average age of the boys is 12 years and that of the girls is 11 years. If the average age of the school is 11 years 9 months, then the number of girls in the school is