math
posted by kirk .
The amount of gold jewelry and other products is measured
in karats (K), where 24K represents pure gold. The mark 14K
on a chain indicates that the ratio between the mass of the
gold in the chain and the mass of the chain is . If a gold
ring is marked 18K and it weighs 0.4 oz, what is the value of
the gold in the ring if pure gold is valued at $300 per oz?

18K gold is 18/24 = 75% gold, by mass.
A 0.4 oz 18K ring therefore contains 0.3 oz of gold.
Multiply 0.3 oz by $300/oz for the gold value in the ring. The other 0.1 oz is probably copper of negligible value.
That is a rather outdated question. Gold is worth $1100/oz these days.