How many grams silver (Ag) is added to 13 grams gold to produce 85% gold (Au) alloy?

Can someone please give me the derived formula to be used? I had managed to solve it though I'm not quite sure if it's correct.

i got 73.67g Ag, can someone please correct me if i'm wrong. Still i don't have an idea how to get the derived formula from this:

grams solute/grams solvent = w/w%

Please help.

73.67 g is right but I would round that to 74. I don't know that there is a way to "DERVIVE" the formula. That's one of those things you learned in the 4th grade arithmetic class. But here is a common sense approach.

If I gave you a basket containing 5 lbs of apples and 5 lbs of oranges, you would tell me the basket was 50% apples and 50% oranges wouldn't you. How did you get that. You said (lbs apples/total weight)*100 = % apples.
So [(lbs anything)/(total lbs)]*100 = % anything w/w