Question 736924
Let {{{ a }}} = liters of 20% solution to be removed
Also, {{{ a }}} = liters of 70% solution to be added
{{{.2a }}} = liters of NaCl removed
{{{ .7a }}} = liters of NaCl added
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{{{ .7a - .2a = .5a }}} liters of NaCl are added
{{{ .2*400 = 80 }}} liters of NaCl to start with
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{{{ ( 80 + .5a ) / 400 = .41 }}}
{{{ 80 + .5a = .41*400 }}}
{{{ .5a = 164 - 80 }}}
{{{ .5a = 84 }}}
{{{ a = 168 }}}
168  liters of 400L of 20% NaCl solution must be 
replaced with 70% NaCl solution
check:
{{{ ( 80 + .5a ) / 400 = .41 }}}
{{{ ( 80 + .5*168 ) / 400 = .41 }}}
{{{  80 + 84 = .41*400 }}}
{{{ 164 = 164 }}}
OK