A cistern can be filled by a tap in 3 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?  Choose one:
A) 4.5 hrs
B) 5 hrs
C) 6.5 hrs
D) 7.2 hrs
E) 9.2 hrs

Rationale: "net part filled in 1 hour = 1 / 3 - 1 / 9 = 2 / 9 therefore the cistern will be filled in 9 / 2 hours or 4.5 hours . answer : a"
Answer: A