A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9hours. If both the taps are opened simultaneously then after how much time will the cistern get filled?	Pick one:
A) 6 hr	B) 5.6 hr	C) 9.5 hr	D) 7.2 hr	E) 4 hr

Rationale: "net part filled in 1 hour 1 / 4 - 1 / 9 = 5 / 36 the cistern will be filled in 36 / 5 hr = 7.2 hr answer is d"
Answer: D