The total number of students in a school was 660. The ratio between boys and girls was 13:9. After some days, 30 girls joined the school and some boys left the school and new ratio between boys and girls became 6: 5. The number of boys who left the school

In the first case,

boys = $\frac{13}{22}\times660$=390

girls = $\frac{9}{22}\times660$=270

If x boys leave the school, then,

$\Rightarrow$\frac{390-x}{270+30}$=$\frac{6}{5}$

=390 – x = 360
= x = 390 – 360 = 30

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