Q:

A cistern can be filled by two pipes filling separately in 12 and 16 min respectively. Both pipes are opened together for a certain time but being obstructions, only 7/8 of full quantity water flows through the former and only 5/6 through the latter pipe. The obstructions, however, being suddenly removed, the cistern is filled in 3 minutes from that moment. How long was it obstructions before the full flow began? 

• 1
  4.5 min
• 2
  2.5 min
• 3
  3.5 min
• 4
  5.5 min