A cistern can be filled by a tap in 3 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?
Pick
A) 5.2 hrs  B) 2.9 hrs  C) 1.9 hrs  D) 7.5 hrs  E) 1.4 hrs

Rationale: "net part filled in 1 hour = ( 1 / 3 - 1 / 5 ) = 2 / 15 the cistern will be filled in 15 / 2 hrs i . e . , 7.5 hrs . answer : d"
Answer: D