Question 1128980
Let {{{ x }}} = liters to be drained out and
replaced with {{{ x }}} liters of pure alcohol
{{{ .25x }}} = liters of alcohol that were removed
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{{{ ( .25*2.5 - .25x + x ) / 2.5 = .4 }}}
{{{ ( .625 + .75x ) / 2.5 = .4 }}}
{{{ .625 + .75x = 1 }}}
{{{ .75x = .375 }}}
{{{ x = .5 }}}
.5 liters must be drained and replaced with pure alcohol
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check:
{{{ ( .625 + .75*.5 )/2.5 = .4 }}}
{{{ ( .625 + .375 ) / 2.5 = .4 }}}
{{{ 1/2.5 = .4 }}}
{{{ 1 = 1 }}}
OK