Question 1031694
Let {{{ x }}} = liters of pure glycol 
to be extracted
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{{{ .79*800 = 632 }}} liters of pure
glycol in 79% solution
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{{{ 800 - x }}} = liters of 79% glycol
solution remaining
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{{{ 632 - x }}} = liters of pure glycol
remaining
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{{{ ( 632 - x ) / ( 800 - x ) = .3 }}}
{{{ 632 - x = .3*( 800 - x ) }}}
{{{ 632 - x = 240 - .3x }}}
{{{ .7x = 392 }}}
{{{ x = 560 }}}
560 liters of pure glycol should be extracted
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check the answer:
{{{ 632 - 560 = 72 }}} liters of pure glycol is left
{{{ 800 - 560 = 240 }}} liters of solution is left
{{{ 72/240 = .3 }}}
OK