the ratio of the number of boys to number of girls in certain school club is 3:4.if there are 5 more girls than boys in the club,how many girls are in the club?
+130511 Let x be the number of boys and x + 5 be the number of girls....and we have....
[x] / [x + 5] = 3 /4 cross-multiply
4x = 3[x + 5] simplify
4x = 3x + 15 subtract 3x from both sides
x = 15 = the number of boys
So....there are 20 girls
+118723 The ratio of the number of boys to number of girls in certain school club is 3:4.if there are 5 more girls than boys in the club,how many girls are in the club?
CPhill has given you a good answer I will talk about a different way.
What this is telling you is that if you split the boys into 3 groups (this will be exact because the number of boy is a multiple of three)
and split the girls into 4 groups (again this will work easily - you won't have to cut a girl in half or anything. 
)
You will have seven groups of children all the groups will have the same number of children. There is just one more group of girls than boys.
We are told that the extra group of girls has 5 students in it, SO every group has 5 students.
So, since there are 4 groups of girls there must be 5*4 = 20 girls.
