Question 1149841
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The combined rate of the pipes A and B is filling  {{{1/3}}} + {{{1/6}}} = {{{3/6}}} = {{{1/2}}}  of the tank volume per hour.


So, after 1 hour working, the two pipes A and B will fill  {{{1/2}}}  of the tank volume.



The combined net inflow rate of the three pipes A, B and C  is  {{{1/3}}} + {{{1/6}}} - {{{1/8}}} = {{{8/24 + 4/24 - 3/24}}} = {{{9/24}}} = {{{3/8}}} of the tank volume per hour.


Hence, the three pipes will fill the remaining  {{{1/2}}}  of the tank volume in  {{{((1/2))/((3/8))}}} = {{{8/6}}} hours = {{{1}}}{{{2/6}}} hours = 

= {{{1}}}{{{1/3}}} hours = 1 hour and 20 minutes.


The total time to fill the tank is then  2 hours and 20 minutes.    <U>ANSWER</U>
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Solved.