A pipe can empty a tank in 40 minutes. A second pipe with diameter twice as much that of the first is also attached with the tank to empty it. The two together can empty the tank in

• 8 minutes
  
  If first pipe that can empty a tank in 40 minutes has radius 'r', its 1 min. empty rate=1/40
  As the second pipe has double the diameter than first will have radius=2r, so its flow will be directly proportional to cross sectional area=Pi*(2r)^2=4*Pi*r^2, which is 4 times the first pipe.
  So the empty rate of second pipe=4/40=1/10
  So together both pipes can empty the tank in 1/[(1/40)+(1/10)]=8 min.
• 8 years agoHelpfull: Yes(3) No(0)
• 1/20+1/40=3/40
  so it will be emptied in 40/3 min.
• 8 years agoHelpfull: Yes(1) No(1)