A cistern can be filled by a tap in 6 hours while it can be emptied by another tap in 12 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?
Select:
A) 5.2 hrs
B) 2.9 hrs
C) 12 hrs
D) 7.2 hrs
E) 1.4 hrs

Rationale: "net part filled in 1 hour = ( 1 / 6 - 1 / 12 ) = 1 / 12 the cistern will be filled in 12 / 1 hrs i . e . , 12 hrs . answer : c"
Answer: C