Answer :

Espeon
2:3=?:12
2:3=2/3
   2           ?
-----  =  -------
   3           12
Cross multiply
2x12=24
3x?=24
24/3=8
There are 8 girls.

Steps:
1. Do a proportion
2/3 times x/12
2. Do 12 times 2=24
3. Divide 24 by 3
4. Your answer is 8 girls
Or you could do it this way
Steps:
1. You could have just multiplied the 2 by a 4 to get 8.12 boys divided by 3 boys is 4. That means you multiple the two by 4 to get the girls.