Question 987142
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Since the first tube can fill the reservoir in  4  hours,  it fills  {{{1/4}}}  of the reservoir volume per hour.

Since the second tube can fill the reservoir in  6  hours,  it fills  {{{1/6}}}  of the reservoir volume per hour.

Working simultaneously,  two tubes fill  {{{1/4 + 1/6}}} = {{{3/12 + 2/12}}} = {{{(3+2)/12}}} = {{{5/12}}}  of the reservoir volume per hour.

Hence,  the reservoir will be filled in  {{{12/5}}} = {{{2}}}{{{2/5}}}  hours,  or  2  hours and  24  minutes,  if both tubes work simultaneously.


<B>Answer</B>. &nbsp;It will take &nbsp;2&nbsp; hours and &nbsp;24&nbsp; minutes to fill the reservoir, &nbsp;if two tubes work simultaneously.



It is a typical problem on joint work. &nbsp;For more of such problems and their solutions see my lesson &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; in this site.