Question 1109160
Let {{{ x }}} = liters of 20% solution needed
{{{ .2x }}} = liters of acid in 20% solution
{{{ .6*8 = 4.8 }}} = liters of acid in 60% solution
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{{{ ( .2x + 4.8 ) / ( x + 8 ) = .5 }}}
{{{ .2x + 4.8 = .5*( x + 8 ) }}}
{{{ .2x + 4.8 = .5x + 4 }}}
{{{ .3x = .8 }}}
{{{ x = 2.667 }}} 
2.667 liters of 20% solution are needed
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check:
{{{ ( .2x + 4.8 ) / ( x + 8 ) = .5 }}}
{{{ ( .2*2.667 + 4.8 ) / ( 2.667 + 8 ) = .5 }}}
{{{ ( .5333 + 4.8 ) / 10.667 = .5 }}}
{{{ 5.333 = .5*10.667 }}}
{{{ 5.333 = 5.333 }}}
OK