A cistern can be filled by a tap in 7 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled ?  Select:
A) 4.5 hrs
B) 5 hrs
C) 6.5 hrs
D) 17.2 hrs
E) 31.5 hrs

Rationale: "net part filled in 1 hour = 1 / 7 - 1 / 9 = 2 / 63 therefore the cistern will be filled in 63 / 2 hours or 31.5 hours . answer : e"
Answer: E