Question 812764
Focus on concentration of Gold (but this is arbitrary).

Let g = grams of Silver to add.


{{{15*75/(15+g)=65}}}


If you could read and analyze the description and obtain that equation, or an equivalent one, then you have solved most of the problem.  The rest is only a few arithmetic steps.


{{{15*15/(15+g)=13}}}
{{{15*15=13(g+15)}}}
{{{15*15=13g+13*15}}}
{{{15(15-13)=13g}}}
{{{30=13g}}}
{{{highlight(g=30/13=2&4/13)}}} grams of silver