First, you need to find out how much will the candle cost per volume. If a candle with a volume of 80cm^3 costs $12, the price of the candle per cm^3 would be: $12/80cm^3= $0.15/cm^3
Then the price of a candle with 120 cm^3 volume would be:120cm^3* $0.15/cm^3= $18