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
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
subtract(multiply(12, 640), multiply(add(11, divide(9, 12)), 640))