Math Help - Adding Concentration Problem

Problem: Joe has a ring weighing 10 grams made of an alloy of 13% silver and the rest gold. He decides to melt down the rings and add enough silver to reduce the gold content to 69%. How many grams of silver should he add?

My set up is:

10 (.13 + .87) = 10 (.13 + X + .69)

I'm not sure if that's correct.

Originally Posted by ayyisei

Problem: Joe has a ring weighing 10 grams made of an alloy of 13% silver and the rest gold. He decides to melt down the rings and add enough silver to reduce the gold content to 69%. How many grams of silver should he add?

My set up is:

10 (.13 + .87) = 10 (.13 + X + .69)

I'm not sure if that's correct.

Let the amount of silver (in gram) added be X.

Then 10 + X = 10(0.87) + (10+X)(0.31).