A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 10 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled? Select the correct option: A) 2.9 hrs B) 8.9 hrs C) 2.9 hrs D) 6.7 hrs E) 8.6 hrs

Rationale: "net part filled in 1 hour = ( 1 / 4 - 1 / 10 ) = 3 / 20 the cistern will be filled in 20 / 3 hrs i . e . , 6.7 hrs . answer : d"
Answer: D