Big school 2

The ratio of the number of boys to the number of girls in a school of 450 pupil is 4 : 5 . When some new boys and girls are admitted, the number of boys increases by 25 and ratio of boys and girls changes to 9 : 13 . Calculate the number of new girls admitted.

Correct answer:

x =  75

Step-by-step explanation:

$$\begin{gathered} n=450 \\ b:g=4:5 \\ b=\frac{4}{4+5}\cdot n=\frac{4}{4+5}\cdot 450=200 \\ g=n-b=450-200=250 \\ (b+25):(g+x)=9:13 \\ (b+25)=9/13\cdot (g+x) \\ (200+25)=9/13\cdot (250+x) \\ 9x=675 \\ x=\frac{675}{9}=75 \\ x=75 \end{gathered}$$

Try another example