If paris has 18 cups of pretzels how many cups of nuts does she need

That depends on how she likes her trail mix . . . heavier on pretzels or
heavier on nuts.  You haven't told us anything about the rule she uses
to prepare her favorite mix.  Personally, I would mix about 300 cups of
nuts with the pretzels, and then before I eat any of it, I would pull out
all the pretzels and give them to my dogs. But that's just me.