Question 65273
One sprinker can fill the fountain in 3 hours 
therefore in 1 hr.  it can fill 1/3 portion of the fountain
 second sprinker can fill the fountain in 6 hrs.
 in1 hr it can fill 1/6 portion 
when both sprinkers operating 
 in 1 hr. they can  1/3 + 1/6 = 1/2 portion of the fountain 
  so  they will take 2 hrs to fill the tab