Question

3/4 of the girls in SSS1 play basket ball and 4/7 play volleyball. Every girl plays at least one of these games. If 27 girls play both games, how many girls are there in the class?

Answer 1

You may wish to start with a Venn diagram : )
That is two intersecting circles... where the overlap is the 27 that play both games.

Answer 2

3/4 x + 4/7 x - 27 = x

Answer 3

20