How many cubic centimeters of ore containing 0.22% by mass gold must be processed...?

How many cubic centimeters of ore containing 0.22% by mass gold must be processed to obtain $100 worth of gold? The density of the ore is 8.0 g/cm³ and the price of gold is $418 per troy ounce. (14.6 troy oz. = 1.0 ordinary pound called an avoirdupois pound, and 1 lb = 454 g.)

C is the answer (4.20 x 10² cm³)

First determine how many grams of gold is equivalent to $100. You will need to converting troy ounce to grams. 1 troy ounce is exactly 31.1034768 grams http://en.wikipedia.org/wiki/Troy_ounce
($418.00 /toz) * (1 toz / 31.1035 grams) = $13.44/gram
Then ($100) / ($13.44/gram) = 7.44 grams of gold to get $100
The the ore is just 0.22% Au, So divide grams of gold needed by this to get grams of ore needed.
(7.44 grams of gold) / (0.0022) = 3382.29 grams of ore needed

Finally, multibly by the density to get cm³ needed:

(3382.29 grams of ore) / (8.0 g/ cm³) = 422 cm³ of ore

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