Question 662322
<pre>
Large pipe's filling rate = {{{(1_tank)/(8_hr)}}} = {{{1/8}}}{{{tank/hr}}}

Small pipe's filling rate = {{{(1_tank)/(x_hr)}}} = {{{1/x}}}{{{tank/hr}}}

Filling rate working together = {{{(1/8+1/x)}}}{{{tank/hr}}}

Part of tank filled in first 3 hours = 3·{{{(1/8+1/x)}}}
Part of tank filled in next 10 hours = 10·{{{(1/x)}}}

      The equation comes from

             {{{(matrix(9,1,

Part,of,tank,filled,in,the,first,3,hours))}}} + {{{(matrix(9,1,

Part,of,tank,filled,in,the,next,10,hours))}}} = {{{(matrix(3,1,

1,tank,filled))}}}

 
            3·{{{(1/8+1/x)}}} + 10·{{{(1/x)}}} = 1 

Solve that and get 20.8 hours. 

Edwin</pre>