Two pipes A and B together can fill a cistern in 4 hours. Had they been opened separately, then B would have taken 6 hours more than A to fill the cistern. How much time will be taken by A to fill the cistern separately?

A. 1 hour
B. 2 hours
C. 6 hours
D. 8 hours

• 
  C. 6 hours
  
  If A takes x hrs, then B takes x+6 hrs
  
  1/x + 1/ (x+6) = 1/4
  
  solving it, we get
  x= 6 hours
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