How Many Girls Excel at Singing but Not Dancing Among the Annie Auditionees?

[tofu]

here it is


Of the 21 young girls auditioning for the role of Annie, 9 were good dancers who couldnot sing well enough, but 5 could sing and dance well. Assuming that all girls possessed at least one of these skills, how many girls must have been good singers w/ out dancing skills

 

[Hurkyl]

What have you tried to do to solve the problem?

(and should this be in homework help?)

 

[rattis]

should be in homework or statistics section.

Are you saying out of 21 girls what is probability that when one is picked at random they are are good at singing but not dancing?