Two pipes can fill a tank in 10 hours and 12 hours respectively while a third pipe empties the full tank in 20 hours.If all the three pipes operate simultaneously, in how much time will the tank be filled?
A. 7 hrs 30 min
B. 7 hrs 45 min
C. 8 hrs 30 min
D. 8 hrs 45 min

• 1/10+1/12-1/20=2/15
  so 15/2 is equal to 7:30
  option A is correct
  
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