Question 720744
Be careful with those bullets- just kidding
Let {{{ x }}} = pounds of lead to be added to mixture
You start out with:
{{{ .3*1 = .3 }}} pounds of tin
{{{ .7*1 = .7 }}} pounds of lead
------------
( total pounds of lead ) / ( total pounds of metal ) = .96
{{{ ( .7 + x ) / ( 1 + x ) = .96 }}}
{{{ .7 + x = .96*( 1 + x ) }}}
{{{ .7 + x = .96 + .96x }}}
{{{ .04x = .96 - .7 }}}
{{{ .04x = .26  }}}
{{{ x = 6.5 }}}
If you add 6.5 pounds of lead, you will have
4% tin, 96% lead
check:
{{{ ( .7 + x ) / ( 1 + x ) = .96 }}}
{{{ ( .7 + 6.5 ) / ( 1 + 6.5 ) = .96 }}} 
{{{ 7.2 / 7.5 = .96 }}}
{{{ 7.2 = .96*7.5 }}}
{{{ 7.2 = 7.2 }}}
OK