Question 1036553
Tap A takes 6 minutes to fill a tank 
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So A's filling rate = 1 tankful per 6 minutes or 1/6 of a tankful per minute.
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and Tap B takes 9 minutes to fill the same tank.
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So B's filling rate = 1 tankful per 9 minutes or 1/9 of a tankful per minute.
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Pipe C can empty the tank in 15 minutes. 
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So C's 'emptying' rate = 1 tankful per 15 minutes or 1/15 of a tankful per minute.

But that's to be interpreted as a negative filling rate.

C's "filling" rate = -1 tankful per 15 minutes or -1/15 of a tankful per minute.
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How long will it take to fill up the tank if the pipe is 
in use when <s>both taps</s> are turned on?
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Both taps?  But there are three pipes mentioned.  Did you 
mean all three pipes were turned on, or just A and B? 

If you like, you can tell me what you meant in the thank-you 
note form below, and I'll get back to you by email.

Edwin</pre>