If 4/15 are males, then you can assume that the rest (11/15) are females.
Find what is 11/15 if 315 using a proportion.
11 f
— = —
15 315
In this proportion, multiply 15 by 21 to get 315, so multiply 11 by 21 to get 231, which is the value of F.
Therefore, f=231. In other words, there are 231 females in the book club.
Hope this helps!