Question 548134
First find out how much of the tank by
pipe A alone
10 AM to noon is 2 hrs
Pipe A's rate is ( 1 tank )/( 6 hrs )
{{{ (1/6)*2 = 1/3 }}} of a tank
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There is 2/3 of the tank left to fill
Let {{{t}}} = time in hrs for B to finish filling tank
B's rate is ( 1 tank ) / ( 8 hrs )
{{{ ( 1/8)*t = 2/3 }}} 
Multiply both sides by {{{24}}}
{{{ 3t = 16 }}}
{{{ t = 5.333 }}}
The time used by both pipes is {{{ 2 + 5.333 = 7.333 }}}
This is 7 hrs and 20 min to fill the tank