A tap can fill a tank in 6 hours.After half the tank is filled, three more similar taps are opened.What is the total time taken to fill in the tank completely???

• first half should be filled in 3hr.and next half should be filled by 4 taps and it should take 3/4 hr.So total time take 3+(3/4)=15/4 hr.
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• using alligation
  
  19 9
  15
  6 4
  6:4=3:2
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• 3hours+20 min
  
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• half tank will be fill in 3 hour
  so remaining 1/2 will be
  1/6+1/6+1/6+1/6=4/6
  or 2/3
  now3/2*1/2=3/4 time
  3/4*60=45 min
  hence 3 hour 45 min
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