Question 911345
Let {{{ x }}} = kg of pure copper to be added
{{{ .12*50 = 6 }}} kg of copper in 12% alloy
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{{{ ( 6 + x ) / ( 50 + x ) = .21 }}}
{{{ 6 + x = .21*( 50 + x ) }}}
{{{ 6 + x = 10.5 + .21x }}}
{{{ .79x = 4.5 }}}
{{{ x = 5.696 }}}
5.696 kg of pure copper must be added
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check:
{{{ ( 6 + x ) / ( 50 + x ) = .21 }}}
{{{ ( 6 + 5.696 ) / ( 50 + 5.696 ) = .21 }}}
{{{ 11.696 / 55.696 = .21 }}}
{{{ 11.696 = .21*55.696 }}}
{{{ 11.696 = 11.696 }}}
OK