Pipe A can fill a tank in 16 minutes and pipe B cam empty it in 24 minutes. If both the pipes are opened together after how many minutes should pipe B be closed, so that the tank is filled in 30 minutes?  Choose the correct choice:
A) 18  B) 27  C) 98  D) 27  E) 21

Rationale: "let the pipe b be closed after x minutes . 30 / 16 - x / 24 = 1 = > x / 24 = 30 / 16 - 1 = 14 / 16 = > x = 14 / 16 * 24 = 21 . answer : e"
Answer: E