A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 11 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?  Pick:
A) 6.24
B) 7.4
C) 7.92
D) 6.28
E) 7.24

Rationale: "net part filled in 1 hour = ( 1 / 4 - 1 / 11 ) = 7 / 44 the cistern will be filled in 44 / 7 hrs i . e . , 6.28 hrs . answer : d"
Answer: D