Question 763949
Let {{{ x }}} = quarts of fluid to be drained and replaced 
with pure antifreeze
{{{ .3x }}} = quarts of pure antifreeze in fluid drained off
{{{ .3*10 = 3 }}} quarts of pure antifreeze originally in fluid
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{{{ ( 3 - .3x  + x ) / 10 = .4 }}}
( note that you start with {{{ 10 }}} quarts and end with {{{ 10 }}} quarts )
{{{ ( 3 + .7x ) / 10 = .4 }}}
{{{ 3 + .7x = 4 }}}
{{{ .7x = 1 }}}
{{{ x = 1.4286 }}}
1.4286 quarts of fluid must be drained off and
replaced with pure antifreeze
check:
{{{ ( 3 - .3x  + x ) / 10 = .4 }}}
{{{ ( 3 - .3*1.4286  + 1.4286 ) / 10 = .4 }}}
{{{  3 - .4286 + 1.4286 = 4 }}}
{{{ 4 = 4 }}}
OK