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Let " T " be the total cost, in dollars.
Then from the condition, you have this equation
    
 - 
 = 9  dollars.
It is direct and literal translation of the condition to Math.
To solve the equation, multiply both its sides by 60. You will get
    3T - 2T = 9*60,   or
    T       = 540  dollars.    ANSWER
Solved.