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
Sol.   Let the number of grils be x.   Then, number of boys = (640 - x).   Then, (11 3/4 × 640)   ⇔ 11x + 12(640 - x) ⇔ x = 7680 - 7520 ⇔ 160.   160nswer 160
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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