A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 8 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?  Choose the most appropriate option:	A) 7.5	B) 7.4	C) 7.9	D) 7.2	E) 8.0

Rationale: "net part filled in 1 hour = ( 1 / 4 - 1 / 9 ) = 1 / 8 the cistern will be filled in 8 / 1 hrs i . e . , 8 hrs . answer : e"
Answer: E