Question 898309
{{{ .3*6 = 1.8 }}} quarts of pure juice 
Let {{{ x }}} = quarts of !00% pure juice 
to be added
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{{{ ( 1.8 + x ) / ( 6 + x ) = .4 }}}
{{{ 1.8 + x = .4*( 6 + x ) }}}
{{{ 1.8 + x = 2.4 + .4x }}}
{{{ .6x = .6 }}}
{{{ x = 1 }}}
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You need 1 quart of 100% pure juice
check:
{{{ ( 1.8 + x ) / ( 6 + x ) = .4 }}}
{{{ ( 1.8 + 1 ) / ( 6 + 1 ) = .4 }}}
{{{ 2.8 / 7 = .4 }}}
{{{ 2.8 = 2.8 }}}
OK