In a school there are 9 boys and 15 girls. 6 more girls joined the school. How many more girls than boys does the school have?
There are 9 boys and 15 girls in the school. 
6 more girls joined the school. 
That means there are now 9 + 6 = 15 girls in the school. 
There are still 9 boys in the school. 
That means there are now 15 - 9 = 6 more girls than boys in the school.