A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 5 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled? Options:  A) 7.5  B) 20  C) 30  D) 32  E) 40

Rationale: "net part filled in 1 hour = ( 1 / 4 - 1 / 5 ) = 1 / 20 the cistern will be filled in 20 / 1 hrs i . e . , 20 hrs . answer : b"
Answer: B