.
Since "It takes Sandra 15 hours to complete a job", she makes of the job per hour.
It is her "rate of work".
Since "Garland can complete the job in 12 hours", he makes of the job per hour.
It is his "rate of work".
Working together, they make + part of the job per hour.
It is their combined rate of work.
+ = = = .
Thus they make of the work per hour. Every hour.
Hence, it will take them hours = hours = 6 hours and 40 minutes to complete the job working together.
Solved.
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It is a 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.