If you were trying to up the carat rating from 9ct to 18ct you would
add an equal amount of 24ct to the 9ct. 14ct to 18ct needs less 24ct.
Say we have 10g of 14ct. 4.15g of that would be alloy material. To
work out how much 24ct you will need to add, you need to make that
4.15g = 25% of the total finished weight. If you add 6.6g of 24ct to
14ct, you end up with a total weight of 16.6g, 25% of that = 4.15g in
alloying material. You now have 18ct (75% gold content)