In a school there are w more girls than boys. If there are x girls How many pupils are there in that school?
First, we know that there are w more girls than boys. This tell us that the number of boys will be w less than the number of girls. 
Secondly, we know that there are x girls. This means that the number of boys will be x - w. 
Therefore, the total number of pupils will be x + (x - w), which is 2x - w.