Question 651004
Let {{{ x }}} = kg of 40% metal needed
{{{ .4x }}} = kg of copper in 40% metal
{{{ .7*60 = 42 }}} kg of copper in 70% metal
{{{ ( .4x + 42 ) / ( x + 60 ) = .5 }}}
{{{ .4x + 42 = .5*( x + 60 ) }}}
{{{ .4x + 42 = .5x + 30 }}}
{{{ .1x = 12 }}}
{{{ x = 120 }}}
120 kg of the 40% metal are needed
check:
{{{ ( .4*120 + 42 ) / ( 120 + 60 ) = .5 }}}
{{{ ( 48 + 42 ) / 180 = .5 }}}
{{{ 90/180 = .5 }}}
{{{ 90 = .5*180 }}}
{{{ 90 = 90 }}}
OK