Pipe A can fill a tank in 6 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) 96
D) 27
E) 21

Rationale: "let the pipe b be closed after x minutes . 30 / 6 - x / 24 = 1 = > x / 24 = 30 / 6 - 1 = 4 = > x = 4 * 24 = 96 . answer : c"
Answer: C