Hi,

old combine, crop can be harvested in 60 hours,

new combine can do the same job in 20 hours.

How many hours would it take to harvest the crop with both combines operating?

PER hour KEY

 1/60 + 1/20 = 1/x  |Multiplying thru by 60x so as all denominators = 1

  x  + 3x = 60

     4x = 60

      x = 15hrs it would take with both combines operating 

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