Question 1056298
Let {{{ x }}} = kg of pure tin to be added
{{{ .38*52 = 19.76 }}} = kg of pure tin in 
the original 52 kg of solder
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{{{ ( x + 19.76 ) / ( x + 52 ) = .5 }}}
{{{ x + 19.76 = .5*( x + 52 ) }}}
{{{ x + 19.76 = .5x + 26 }}}
{{{ .5x = 6.24 }}}
{{{ x = 12.48 }}}
12.48 kg of pure tin must be added
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check:
{{{ ( x + 19.76 ) / ( x + 52 ) = .5 }}}
{{{ ( 12.48 + 19.76 ) / ( 12.48 + 52 ) = .5 }}}
{{{ 32.24 / 64.48 = .5 }}}
{{{ 32.24 = 32.24 }}}
OK