question_answer

In what time would a cistern be filled by three pipes whose diameters are 1cm, 2 cm and 3 cm running together, when the largest pipe alone can fill the tank in 21 minutes. The amount of water flowing through the pipe is directly proportional to the square of its diameter?

A) 10.5 minutes

B) 11.5 minutes

C) 12.5 minutes

D) 13.5 minutes

E) None of these