Question 754802
Let {{{ x }}} = liters of water to be evaporated
{{{ .6*200 = 120 }}} liters of vinegar in 60% solution
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{{{ 120 / ( 200 - x ) = .9 }}}
{{{ 120 = .9*( 200 - x ) }}}
{{{ 120 = 180 - .9x }}}
{{{ .9x = 180 - 120 }}}
{{{ .9x = 60 }}}
{{{ x = 66.667 }}} 
66.667 liters of water should be evaporated
check:
{{{ 120 / ( 200 - x ) = .9 }}}
{{{ 120 / ( 200 - 66.667 ) = .9 }}}
{{{ 120 / 133.333 = .9 }}}
{{{ 120 = 120 }}}
OK