The ratio of Mary's age to Alice's  age is  $3:5$. Alice is 30 years old. How many years old is Mary?
I need to find Mary's age, which I'll call M. I can write this as a proportion: $\frac{M}{30} = \frac{3}{5}$. To solve for M, I can cross-multiply and get: $5M = 90$. Then I can divide both sides by 5 and get: $M = 18$. So Mary is 18 years old. 