Answer:
225
Explanation:
Let G and B be the number of girls and boys in 2012.
The ratio boys to girls was 3 to 7.
This means
B/G = 3/7
There were 300 more girls than boys in 2012.
This means
G = B+300
Replace in the first equation and solve for B
B/(B+300) = 3/7 ===> 7B = 3(B+300) ===> 7B = 3B + 900 ===>
===> 7B-3B = 900 ===> 4B = 900 ===> B=900/4 ===> B=225