In a club there are 1500 boys and some girls, the ratio of number of girls to the boys is 3:10. after some more girls joined the club, the ratio of number of girls to the number of boys becomes 9:15. Find the innitial number of girls in the club.
If there are 3 girls for every 10 boys let $B=10x=1500$ so $x=150$. Thus $G=3x=450$. The second part is not needed to find the initial number of girls but it is a similar problem. If the new ratio is $9:15$, then $B=15y=1500$ and $y=100$ so $G=9x=900$ meaning that the girl population has doubled.