| In a school with 600 students, the average age of the boys is 12 yr and that of the girls is 11 yr. If the average age of the school is 11 yr and 9 months, then the number of girls in the school is [SSC (CGL) 2012] |
A) 450
B) 150
C) 250
D) 350
Correct Answer: B
Solution :
| Let the number of girls be x. |
| Number of boys \[=600-x\] |
| According to the question, |
| \[(600-x)\times 12+11x=11\frac{9}{12}\times 600\] |
| \[\Rightarrow \] \[(600-x)\times 12+11x=\frac{47}{4}\times 600\] |
| \[\Rightarrow \] \[7200-12x+11x=\frac{47}{4}\times 600\] |
| \[\Rightarrow \] \[x=7200-7050=150\] |
| Hence, the number of girls\[=150\] |
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