Question

The ratio of the number of girls to the number of boys in a school is 2:5. if the number of boys is 225, find

Answer 1

I think that the purpose is to find the number of girls. 

Using the ratio and proportion method. 
Ratio of number of girls to the number of boys = 2:5
Number of boys = 225
Let the number of girls be $x$. 

$2:5::x:225$
$\frac{2}{5} = \frac{x}{225}$
Cross multiply. 
$5 * x = 225 * 2$
$5x = 450$
$\frac{5x}{5} = \frac{450}{5}$
$x = 90$
∴ The number of girls in the school is 90. 

Hope this may help you.