Question 1098651
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See the lesson

&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> 

in this site.


Your problem is exactly Problem 5 of this lesson.


You will find there many other similar solved problems with detailed explanations.