To find out how many minutes it took Liam to travel one mile, we can set up the unit rate based on the information given.
Liam took 15 minutes to ride 3 miles. To find the time per mile, we divide the total time by the number of miles:
\[ \text{Time per mile} = \frac{15 \text{ minutes}}{3 \text{ miles}} = 5 \text{ minutes per mile} \]
Therefore, the answer is 5 minutes/mile.