Posted by **missy** on Sunday, June 26, 2011 at 10:21pm.

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.

- Math Concentration -
**Henry**, Tuesday, June 28, 2011 at 5:18pm
Silver = 0.13 * 10 = 1.3 grams.

Gold = 10 1.3 = 8.7 grams.

New Alloy: Gold=69% of total solution.

G / (G+S) = 0.69,

8.7 / (8.7 + S) = 0.69,

Cross multiply:

6 + 0.69S = 8.7,

0.69S = 8.7 - 6 = 2.7,

S = 3.91 Grams of silver, total.

3.91 - 1.3 = 2.6 grams of silver added.

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