Question 613715
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Hi, 
Bob takes 3 hours to paint a room  and Linda 5 hours to paint a room
Let x represent the time it would take them together
PER hour is KEY
 1/3 + 1/5 = 1/x        |Multiplying thru by 15x so as all denominators = 1
   5x + 3x = 15
     8x = 15
      x = 15/8 hrs  or 2 7/8 hrs