In a school there are w girls and x boys. y more girls and z more boys joined the school. How many girls are there in the school now?
There are w girls and x boys in the school. 
y more girls joined the school. 
That means there are w + y girls in the school. 
z more boys joined the school. 
That means there are x + z boys in the school. 

To find out how many students there are in total, we can add w + y + x + z. 
To find out how many girls there are in total, we can add w + y.