Question

I start with 5 oz. of 18-karat gold. It's 75% pure gold, 25% other metals. I need to make it 22-karat, which is 91.7% pure gold. How much pure gold do I need to add to make it 22-karat?

Answer 1

Answer:

about 10.06 oz.

Step-by-step explanation:

Let x represent the number of ounces of pure gold you need to add. Then the amount of gold in the mix is ...

100%·x + 75%·5 = 91.7%·(x+5)

8.3%·x = 5·16.7% . . . . . . subtract 91.7%·x +75%·5

x = 5 · 16.7/8.3 . . . . . . . . divide by the coefficient of x

x ≈ 10.06 . . . . oz

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Alternate solution

The amount of non-gold in the given material is 25%·5 oz = 1.25 oz. That is allowed to be 8.3% of the final mix, so the weight of the final mix will be ...

(1.25 oz)/0.083 ≈ 15.06 oz

Since that weight will include the 5 oz you already have, the amount of pure gold added must be ...

15.06 oz - 5 oz = 10.06 oz

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Comment on these answers

If you work directly with carats instead of percentages, you find the amount of pure gold you need to add is 10.00 ounces, double the amount you have.