.
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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Their combined rate of work is of the job per hour.
Jill's individual rate of work is of the job per hour.
Hence, Jessica's individual rate of work is the difference
- = - = = = =
of the job per hour.
It means that Jessica can complete the job in = 15 hours = 15 hours and 45 minutes. ANSWER
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. See the lessons
- Using Fractions to solve word problems on joint work
- Solving more complicated word problems on joint work
- Selected joint-work word problems from the archive
Read them and get be trained in solving joint-work problems.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this textbook under the topic
"Rate of work and joint work problems" of the section "Word problems".
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.