Question 783163
Let {{{ x }}} = liters to be drained
{{{ .3x }}} = liters of chemical in {{{ x }}} liters of solution
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{{{ x }}} liters of 30% solution are replaced with {{{ x }}}
liters of 70% solution 
{{{ .7x }}} = liters of chemical in 70% solution
Note that total liters of solution doesn't change
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{{{ .3*80 = 24 }}} liters of chemical in {{{ 80 }}} liters of solution
{{{ ( 24 - .3x + .7x ) / 80 = .4 }}}
{{{ ( 24 + .4x ) / 80 = .4 }}}
{{{ 24 + .4x = 32 }}}
{{{ .4x = 8 }}}
{{{ x = 20 }}}
20 liters should be drained & replaced with 70% solution
check:
{{{ ( 24 - .3x + .7x ) / 80 = .4 }}}
{{{ ( 24 - .3*20 + .7*20 ) / 80 = .4 }}}
{{{ ( 24 - 6 + 14 ) / 80 = .4 }}}
{{{ 32/80 = .4 }}}
{{{ 32 = 32 }}}
OK