Question 1183385
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I do not think that in this problem they assume that you are familiar with the basics of hydraulics

to make a conclusion that the flow rate through the pipe of the doubled diameter is 4 times 

the flow rate through the narrow pipe.



Simply, you did not read the problem to the end and posted the problem in the incomplete mode.



I am 179% sure that the next phrase in the problem, which you missed to read and to print, is



                How fast the second pipe can empty the tank, if it works alone ?



The info about flow rate is not intended for you to find out "why ?"


It is simply the  "given part",   after which the question follows.



On solving such problems, look the lessons

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Rate-of-work-word-problems/Word-problems-WORKING-TOGETHER-by-Fractions.lesson>Using Fractions to solve word problems on joint work</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=http://www.algebra.com/algebra/homework/Rate-of-work-word-problems/Solving-more-complicated-word-problems-on-joint-work.lesson>Solving more complicated word problems on joint work</A> 

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/Rate-of-work-word-problems/Selected-problems-from-the-archive-on-joint-work-word-problems.lesson>Selected joint-work word problems from the archive</A> 



Read them and get be trained in solving joint-work problems.



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Next time, when you post your problems/questions to this forum, 


print and post them &nbsp;IN &nbsp;FULL, &nbsp;word-in-word, &nbsp;from the very beginning to the very end,


INCLUDING &nbsp;the &nbsp;QUESTION &nbsp;itself and the question sign after it . . . 



These are the RULES of this forum, 

and you just violated them in your post.



About the rules of the forum, see this Internet page


https://www.algebra.com/tutors/students/ask.mpl?action=ask_question&topic=Equations&return_url=http://www.algebra.com/algebra/homework/equations/


from which you post your problems.



It is assumed that you read these rules before posting.


It is also assumed that you do understand what is written in that page and follow the rules.



Those who violate them, &nbsp;work against their own interests.



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After observing all arrived answers, I become even more certain, than I was certain before,

that the only right strategy to treat such posts is to delete them.