Set up an equation. Since the ratio is 3:4 male:female, it would be 3m=4f. Since you know that 3m=18, solve for m and plug it into the 3m=4f equation to get the number of females! Don't forget that the number of females is 4f not just f!
Discrete domains and continuous domains are both sets. However a discrete domain contains a finite number of elements and continuous can contain and infinite number of elements.
