Hi

Let x represent the time together

Questions States***  Per hour being the equalizer...

 1/6 + 1/9 = 1/x  Multiplying each term by 36x so as all denominators = 1

  6x + 4x = 36

     10x = 36 

       x = 3.6hrs



CHECKING our Answer***

 6/36 + 4/36 = 10/36 =  1/ 36/10  

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