2006 AMC 10A Problem 3

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