Question 831330
Let {{{ x }}} = liters of pure alcohol needed
{{{ ( x + .15*5 ) / ( x + 5 ) = .25 }}}
{{{ x + .75 = .25*( x+5 ) }}}
{{{ x + .75 = .25x + 1.25 }}}
{{{ .75x = .5 }}}
{{{ x = .667 }}} liters
2/3 of a liter of pure alcohol must be added
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check:
{{{ ( x + .15*5 ) / ( x + 5 ) = .25 }}}
{{{ ( .667 + .15*5 ) / ( .667 + 5 ) = .25 }}}
{{{ .667 + .75 = .25*5.667 }}}
{{{ 1.417 = 1.4168 }}}
close enough