AMC10 2006 10A, Problem 3

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

Solution: Let $m$ be Mary's age. Then $\frac{m}{30}=\frac{3}{5}$. Solving for $m$, we obtain $m=\fbox{18}.$

Answer: 18