Question 1012188
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Rob takes 9 hours to clean the attic. 
Huong can clean the same attic in 15 hours. 
How long would it take if they worked together? 
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<pre>
Rob makes   {{{1/9}}} of the job in one hour.
Huong makes {{{1/15}}} of the job in one hour.
Working together, they make {{{1/9 + 1/15}}} = {{{5/45 + 3/45}}} = {{{8/45}}} of the job in one hour.
Hence, it will take {{{45/8}}} = {{{5}}}{{{5/8}}} hours for Rob and Huong to make the job working together.
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See the lesson <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 for more solved problems on joint work.