Question 1062376
Originally, gold/aloy = 20/60 = {{{ 1/3 }}}
4 ounces of the alloy has
{{{ 4*(1/3) = 4/3 }}} ounce of gold
{{{ 4*(2/3) = 8/3 }}} ounces of alloy
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{{{ ( 4/3 ) / (( 8/3 + x )) = 1/4 }}}
{{{ 4/3 = (1/4)*( 8/3 + x ) }}}
{{{ 4/3 = 8/12 + (1/4)*x }}}
multiply both sides by {{{ 12 }}}
{{{ 16 = 8 + 3x }}}
{{{ 3x = 8 }}}
{{{ x = 8/3 }}}
2.667 ounces of copper must be added
to the 4 ounces of alloy
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check:
{{{ ( 4/3 ) / (( 8/3 + x )) = 1/4 }}}
{{{ ( 4/3 ) / (( 8/3 + 8/3 )) = 1/4 }}}
{{{ 4/16 = 1/4 }}}
{{{ 1/4 = 1/4 }}}
OK
Check the math & get another opinion, too