Question

How many ounces of pure gold are in an 18 karat gold chain that weighs 2.8 ounces?

Answer 1

Pure gold is 24 Karat's so the ratio of purity 18 karat gold is:
(Number of Karats of Gold chain)/(Number of Karats of pure Gold)
this will give us:
18/24
=3/4 
The weight of pure gold in 18 Karat Gold will be:
(ratio of purity)×(Total weight of chain)
=3/4×18
=13 1/2 ounces
=13.5 ounces.

Answer 2

18 karat gold contains 75% gold, and 25% other metals in the alloy. 75% of 2.8 ounces yields a gold content of 2.1 ounces. Multiplying .75 times 2.8 will yield the correct result.