A cistern can be filled by a tap in 5 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 the correct option.
A) 4.5 hrs	B) 5 hrs	C) 6.5 hrs	D) 7.25 hrs	E) 11.25 hrs

Rationale: "net part filled in 1 hour = 1 / 5 - 1 / 9 = 4 / 45 therefore the cistern will be filled in 45 / 4 hours or 11.25 hours . answer : e"
Answer: E