Question 1183385
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The rate at which water flows through the pipes is proportional to the cross sectional area of the pipes.  Since the diameter of the second pipe is 2 times the diameter of the first, water flows through the second pipe at a rate 2^2=4 times as fast as through the first pipe.<br>
So having both pipes working is like having 1+4=5 of the first pipes.  Then, since the first pipe can empty the tank in 40 minutes, the two pipes together can empty it in 40/5 = 8 minutes.<br>
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Note that a couple of tutors who responded to this post failed to understand what the reader was asking for.<br>
He KNOWS the answer is 8 minutes; what he is clearly asking for is help to understand HOW to get that answer.<br>
My response above and the one from tutor @Math_helper both do that, in different ways....<br>