In a school there are w boys and x girls. y more girls joined the school. How many more girls than boys does the school have?
Let's denote the number of boys by b and the number of girls by g.

b = w
g = x

Now we know that there are w boys and x girls in the school. 

y more girls joined the school. So now there are x + y girls in the school. 

Therefore, the answer (symbolic expression) is: 

g + y - b