.
This problem is for MENTAL solution.
$20 for every two hours at the office plus $12 dollars for every hour on the tractor is 20+12 = 32 dollars 
              for every quantum ( 2 h office + 1 h tractor) of the work.
In all, there are  = 13 such quantums  ====>  2*13 = 26 hours at the office and 13 hours on the tractor.
Solved.
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Dear "students" !
Today is a special day.
Today I solved  16  problems for you and got back  0  (zero,  ZERO)  "thanks"  from you.
It is the absolute record ! 
    It is just out of limits of my understanding  !
        I never saw it before  !
Thank you very much and wish you to think:   WHO ARE YOU . . .