Melting 9ct and 18ct gold together

so you need 5x5/4 gms = 6.25gms of 18ct If you don't like the
maths, the final formula is easy enough: the ratio of hi carat to
lo carat is equal to the wanted carat minus the lo carat divided by
the hi carat minus the wanted carat. 

The easy answer is however much 9ct you have, 1.25 grams of 18ct for
every gram of 9ct.

Or inversely 0.8 grams of 9ct for every gram of 18ct.

14/24 = .583333. In the US we like it to be.585, so just a little
more of the high carat would be better. Play it safe and go to 1.3
grams of 18ct for every gram of 9ct you would get .587