A cistern can be filled by a tap in 5 hours while it can be emptied by another tap in 6 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled ?	Choose the correct choice from the following answers:
A) 4.5 hrs
B) 5 hrs
C) 6.5 hrs
D) 7.2 hrs
E) 30 hrs

Rationale: "net part filled in 1 hour = 1 / 5 - 1 / 6 = 1 / 30 therefore the cistern will be filled in 30 hours . answer : e"
Answer: E