Question 944486
Let the gold coin's weight be represented by C.
You are told the coin is 56% gold. That gold is then melted with 7g of pure gold. So how much many grams does the coin weigh if the total at the end is 67% gold by weight?

{{{(C*0.56) + (7*1) = (7+C)* 0.67}}}
{{{0.56C + 7 = (7*0.67) + 0.67C}}}
{{{7 - (7*0.67) = 0.67C - 0.56C}}}
{{{7 - 4.69 = 0.11C}}}
{{{2.31/0.11 = C}}}
{{{21=C}}}

So the original coin weighs 21 grams