Question 1028646
Let {{{ x }}} =pounds of metal in the 80% copper barrel
{{{ 120 - x }}} = pounds of metal in the 40% copper barrel
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{{{ ( .8x + .4*( 120 - x )) / 120 = .48 }}}
{{{ ( .8x + 48 - .4x ) / 120 = .48 }}}
{{{ .4x + 48 = .48*120 }}}
{{{ .4x = 57.6 - 48 }}}
{{{ .4x = 9.6 }}}
{{{ x = 24 }}}
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24 pounds of metal were in the barrel 
that contained the 80% copper
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check:
{{{ ( .8x + .4*( 120 - x )) / 120 = .48 }}}
{{{ ( .8*24 + .4*( 120 - 24 )) / 120 = .48 }}}
{{{ ( 19.2 + .4*96 ) / 120 = .48 }}}
{{{ ( 19.2 + 38.4 ) / 120 = .48 }}}
{{{ 57.6 / 120 = .48 }}}
{{{ 57.6 = 57.6 }}}
OK