Question 1040135
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A large pump can fill a tank in 28 minutes. Both a large pump and a small pump can fill a tank in 20 minutes. 
How long would it take the small pump to fill the tank by itself? 
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Two pumps working together fill  {{{1/20}}}  of the tank volume per minute.


One pump (large) fills  {{{1/28}}}  of the tank volume per minute.


Hence, the second pump fills  {{{1/20 - 1/28}}} = {{{7/140 - 5/140}}} = {{{(7-5)/140}}} = {{{2/140}}} = {{{1/70}}}  of the tank volume per minute.


It means that it will take 70 minutes for the second pump to fill the tank working alone.



      Lesson to learn from this solution:  use rates of work.
      You can add and distract them.  It does make sense.



For many other joint-of-work problems see 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> 

in this site.