Answer:
15 girl
b = g-7
To answer this question, you need to know how to translate the description into an equation. In this case, there is two equation which was: (g=girl, b=boy)
1. g=b+7
2. g+b=23
The 1st equation can be changed into b=g-7 which was express the number of the boy. If you insert this equation into second you will got the number of girls
g+b=23
g + (g -7)=23
2g= 30
g= 15
