The ratio of Mary's age to Alice's  age is  $3:5$. Alice is 30 years old. How many years old is Mary?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
The ratio of Mary's age to Alice's  age is  $3:5$. Alice is 30 years old. How many years old is Mary?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
18
The ratio of Mary's age to Alice's  age is  $3:5$. Alice is 30 years old. How many years old is Mary?
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