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4.Average
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In a school with $600$ 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 $11 \,years$ and $9 \,months,$ then the number of girls in the school is
A
$450$
B
$150$
C
$250$
D
$350$
Solution
Number of girls $=x$
Number of boys $=600-x$
$\therefore \quad(600-x) \times 12+11 x$
$=11 \frac{3}{4} \times 600=\frac{47}{4} \times 600$
$\Rightarrow 7200-12 x+11 x=7050$
$\Rightarrow \quad x=7200-7050=150$
Std 13
Quantitative Aptitude
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