Question 943103
The exercise is no different than any of these:


<a href="http://www.algebra.com/my/Mixtures%3A-All-in-Symbols.lesson?content_action=show_dev">Mixing two concentrations for known amount of mixture </a>
http://www.algebra.com/my/Mixtures%3A-All-in-Symbols.lesson?content_action=show_dev


The concentration can simply be in unit, karats or {{{numerator/24}}}.  You could also convert the karats into percents if you want that way.  12 karats would be 50% gold.