If [tex]s[/tex] is the amount of pure silver you add to the alloy, then the new alloy will have a total weight of [tex]15+s[/tex] oz. You want 92.5% of this alloy to be silver, so that the amount of silver in the new alloy is [tex]0.925(15+s)[/tex].
80% of the starting 15 oz alloy was silver; this comes out to 12 oz. To this amount we are adding [tex]s[/tex]. So
[tex]0.925(15+s)=12+s[/tex]
and solving this tells you that you need to add [tex]s=25[/tex] oz of silver to get the sterling silver alloy.
