Question 872545
Add the rate of filling and subtract
the rate of emptying
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Rate of filling:
( 1 tank filled ) / ( 4 hrs )
Rate of emptying:
( 1 tank ) / ( 6 hrs )
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( 1 tank filled ) / ( t hrs ) is the rate
of filling with both pipes open
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{{{ 1/4 - 1/6 = 1/t }}}
Multiply both sides by {{{ 12t }}}
{{{ 3t - 2t = 12 }}}
{{{ t = 12 }}}
It takes 12 hrs  for the tank tank to 
fill with both pipes operating