2006 AMC 12A Problem 3

The ratio of Mary's age to Alice's age is 3:5. Alice is 30 years old. How old is Mary?

\mathrm{(A) \ } 15\qquad \mathrm{(B) \ } 18\qquad \mathrm{(C) \ } 20\qquad \mathrm{(D) \ } 24\qquad \mathrm{(E) \ } 50

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.