Question 511241
In words:
( final quarts of antifreeze) / ( final quarts of fluid ) = 55%
Let {{{ x }}} = quarts of fluid to be drained 
and replaced with pure antifreeze
given:
{{{ .4*9 = 3.6 }}} = quarts of antifreeze in original  mixture
{{{ .4x }}} = quarts of antifreeze that is drained off
------------
{{{ ( 3.6 - .4x + x ) / 9 = .55 }}}
{{{ ( 3.6 + .6x ) / 9 = .55 }}}
{{{ 3.6 + .6x = 4.95 }}}
{{{ .6x = 4.95 - 3.6 }}}
{{{ .6x = 1.35 }}}
{{{ x = 2.25 }}}
2.25 quarts of fluid should be drained 
and replaced with pure antifreeze
check:
{{{ ( 3.6 + .6*2.25 ) / 9 = .55 }}}
{{{ ( 3.6 + 1.35 ) / 9 = .55 }}}
{{{ 4.95 / 9 = .55 }}}
{{{ 4.95 = 4.95 }}}
OK