Question 525770
let {{{x }}} = liters of 20% solution needed
given:
{{{ .9*60 = 54 }}} liters of alcohol in 90% solution
{{{ .2x }}} = liters of alcohol in 20% solution
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{{{ ( 54 + .2x ) / ( 60 + x ) =  .7 }}}
{{{ 54 + .2x = .7*( 60 + x ) }}}
{{{ 54 + .2x = 42  + .7x }}}
{{{ .5x = 12 }}}
{{{ x = 24 }}}
24 liters of 20% solution are needed
check answer:
{{{ ( 54 + .2x ) / ( 60 + x ) =  .7 }}}
{{{ ( 54 + .2*24 ) / ( 60 + 24 ) =  .7 }}}
{{{ ( 54 + 4.8 ) / 84 = .7 }}}
{{{ 58.8 = .7*84 }}}
{{{ 58.8 = 58.8 }}}
OK