A cistern can be filled by a tap in 2 hours while it can be emptied by another tap in 4hours. If both the taps are opened simultaneously then after how much time will the cistern get filled?
Select the correct option:
A) 2 hr B) 5 hr C) 7 hr D) 1 hr E) 4 hr

Rationale: "net part filled in 1 hour 1 / 2 - 1 / 4 = 1 / 4 the cistern will be filled in 4 hr answer is e"
Answer: E