Math Concentration
posted by missy .
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
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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