There are two pipes in a tank. Pipe A is for filling the tank and Pipe B is for emptying the tank. If A can fill the tank in 10 hours and B can empty the tank in 15 hours then find out how many hours it will take to completely fill a half empty tank?
A) 30 hours B) 15 hours C) 20 hours D) 33.33 hours

• Time taken to completely fill a half empty tank is 15 hours
  
  
  Step-by-step explanation:
  
  Given,
  
  A can fill a tank in 10 hours
  
  B can empty a tank in 15 hours
  
  Part of tank filled by A in 1 hour =
  
  Part of tank emptied by B in 1 hour =
  
  Net part of tank filled in 1 hour = -
  
  =
  
  =
  
  =
  
  Therefore,
  
  Both a and b can fill tank in 30 hrs
  
  According to question,
  
  Tank should be half empty
  
  So time taken to fill the half tank =   = 15
  
  Thus , Time taken to completely fill a half empty tank is 15 hours
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