Question 1057753
Let {{{ x }}} = ounces of 30% copper alloy needed
{{{ ( .3x + .9*( 300 ) ) / ( x + 300 )  = .4 }}}
{{{ ( .3x + 270 ) / ( x + 300 ) = .4 }}}
{{{ .3x + 270 = .4*( x + 300 ) }}}
{{{ .3x + 270 = .4x + 120 }}}
{{{ .1x = 150 }}}
{{{ x = 1500 }}}
1,500 ounces of 30% copper alloy are needed
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check:
{{{ ( .3x + .9*( 300 ) ) / ( x + 300 )  = .4 }}}
{{{ ( .3*1500 + .9*( 300 ) ) / ( 1500 + 300 )  = .4 }}}
{{{ ( 450 + 270 ) / 1800 ) = .4 }}}
{{{ 720 / 1800 = .4 }}}
{{{ 720 = .4*1800 }}}
{{{ 720 = 720 }}}
OK