Question 703384
Let {{{ x }}} = liters of mixture to be drained 
and replaced with pure antifreeze
At the start,
{{{ .4*15 = 6 }}} liters of pure antifreeze in 40% mixture
{{{ 4x }}} = liters of pure antifreeze in mixture drained
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{{{ ( 6 - .4x + x ) / 15 = .5 }}}
{{{ ( 6 + .6x ) / 15 = .5 }}}
{{{ 6 + .6x = .5*15 }}}
{{{ .6x = 7.5 - 6 }}}
{{{ .6x = 1.5 }}}
{{{ x = 1.5 / .6 }}}
{{{ x = 2.5 }}}
2.5 liters of mixture must be drained 
and replaced with pure antifreeze 
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check:
{{{ ( 6 - .4*2.5 + 2.5 ) / 15 = .5 }}}
{{{ ( 8.5 - 1 ) / 15 = .5 }}}
{{{ 7.5 = .5*15 }}}
{{{ 7.5 = 7.5 }}}
OK