How to solve modeling problem solving question?

Question 1 asks: At a fun park, a motorized toy boat operates 5 minutes for every dollar coin placed in a meter. The meter will accept a maximum of 120 one dollar coins, write a rule which gives the time of boat operation, B hours, in terms of the number of dollar coins. (the answer says B = n/12) can you please explain step by step? Because I'm unsure as to why you have to divide 120 by 2 first?