Well, I've finished it, but I only have four 24/9's and five 26/15's (despite what I wrote earlier) ... or it may be the other way around! My understanding is that you HAVE to change X to Y (or vice versa) to fit the answer into the grid to agree with previous answers, such that the resulting new word is a valid word. And I am still three words short. Apart from changing some X's or Y's to make the total correct (and I see no reason to do that unnecessarily), there seems little else I can do.