Question 1180590
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Working together, Jessica and Jill can install a new deck in 9 hours. 
Had she done it alone it would have taken Jill 21 hours.
How long would it take Jessica to do it alone?
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<pre>
Their combined rate of work is  {{{1/9}}}  of the job per hour.


Jill's individual rate of work is  {{{1/21}}}  of the job per hour.


Hence, Jessica's individual rate of work is the difference


    {{{1/9}}} - {{{1/21}}} = {{{21/(9*21)}}} - {{{9/(9*21)}}} = {{{(21-9)/(9*21)}}} = {{{12/(9*21)}}} = {{{4/(3*21)}}} = {{{4/63}}}


of the job per hour.


It means that Jessica can complete the job in  {{{63/4}}} = 15 {{{3/4}}}  hours = 15 hours and 45 minutes.    <U>ANSWER</U>
</pre>

Solved.


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It is a standard and typical joint work problem.


There is a wide variety of similar solved joint-work problems with detailed explanations in this site. &nbsp;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> 



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


Also, &nbsp;you have this free of charge online textbook in ALGEBRA-I in this site

&nbsp;&nbsp;&nbsp;&nbsp;- <A HREF=https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson>ALGEBRA-I - YOUR ONLINE TEXTBOOK</A>.


The referred lessons are the part of this textbook under the topic 
"<U>Rate of work and joint work problems</U>" &nbsp;of the section &nbsp;"<U>Word problems</U>".



Save the link to this online textbook together with its description


Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson


to your archive and use it when it is needed.