A cistern can be filled by a tap in 2 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled ? Answers.
A) 2.57 hrs  B) 5 hrs  C) 6.57 hrs  D) 7.2 hrs  E) 9.27 hrs

Rationale: "net part filled in 1 hour = 1 / 2 - 1 / 9 = 7 / 18 therefore the cistern will be filled in 18 / 7 hours or 2.57 hours . answer : a"
Answer: A