Question 658364
Let {{{ x }}} = liters of 10% alcohol solution needed
{{{ .1x }}} = liters of alcohol in 10% solution
{{{ .8*70 = 56 }}} liters of alcohol in 80% solution
given:
{{{ ( .1x + 56 ) / ( x + 70 ) = .6 }}}
{{{ .1x + 56 = .6*( x + 70 ) }}}
{{{ .1x + 56 = .6x + 42 }}}
{{{ .5x = 14 }}}
{{{ x = 14/.5 }}}
{{{ x = 28  }}}
28 liters of 10% alcohol solution are needed 
check:
{{{ ( .1x + 56 ) / ( x + 70 ) = .6 }}}
{{{ ( .1*28 + 56 ) / ( 28 + 70 ) = .6 }}}
{{{ ( 2.8 + 56 ) / 98 = .6 }}}
{{{ 58.8 / 98 = .6 }}}
{{{ 58.8 = .6*98 }}}
{{{ 58.8 = 58.8 }}}
OK