In a school with 640 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 one.
A) 160  B) 200  C) 250  D) 350  E) none

Rationale: sol . let the number of grils be x . then , number of boys = ( 640 - x ) . then , ( 11 3 / 4 × 640 ) ⇔ 11 x + 12 ( 640 - x ) ⇔ x = 7680 - 7520 ⇔ 160 . answer a
Answer: A