In a school there are w more girls than boys. If there are x girls How many pupils are there in that school?
There are w more girls than boys. 

That means that if we start with the number of boys, and then add w to it, we will get the number of girls. 

If we start with the number of boys, and then add w to it, and then add x to that, we will get the total number of students in the school. 

That means that x + (x + w) = the total number of students in the school. 

We can solve for x: 

x + w + x = the total number of students in the school

2x + w = the total number of students in the school

2x = the total number of students in the school - w

2x = the total number of students in the school - (w more girls than boys)

2x = the total number of students in the school - w more 

2x = the total number of students in the school - w more students 

2x = the total number of students in the school - w more students than what? 

2x = the total number of students in the school - w more students than there are boys in