A cistern can be filled by one of two pipes in 30 minutes and by the other in 36 minutes. Both pipes are opened together for a certain time but being particularly clogged only 5 / 6 of the full quantity of water flows through the former and only 9 / 10 through the later. The obstructions, however, being suddenly removed the cistern is filled in 15 1/ 2 minutes from that moment. How long was it before the full flow of water began?

• 1 minutes
• 2 minutes
• 21/2 minutes
• 11/2 minutes