Please reason step by step, and put your final answer within \boxed{}.
A boat took 30 minutes to travel from one dock to another (it's not known which one) and 18 minutes to travel from the second dock to the third. How many minutes will it take to travel from the third dock to the first? (The speed of the river current is constant and the same throughout all its parts. The boat's own speed is also constant.)
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