Question 1070834
Let {{{ x }}} = quarts of fluid to be drained
and replaced 
{{{ .15x }}} = quarts of antifreeze in the fliuid
that is drained and replaced
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{{{ ( .15*( 6 - x ) + x ) / 6 = .45 }}}
{{{ ( .9 - .15x + x ) / 6 = .45 }}}
{{{ ( .85x + .9 ) / 6 = .45 }}}
{{{ .85x + .9 = 2.7 }}}
{{{ .85x = 1.8 }}}
{{{ x = 2.1176 }}}
2.12 quarts of fluid must be drained and replaced
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check:
{{{ .15*6 = .9 }}} ounces of antifreeze to start with 
{{{ .15*2.12 = .318 }}} ounces of antifreeze is drained off
{{{ 2.12 }}} ounces of antifreeze is added
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{{{ .9 - .318 + 2.12 = 2.702 }}} final ounces of antifreeze
{{{ 2.702/6 = .45 }}}
{{{ 2.702 = 2.7 }}}
close enough
check the math
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