A: We consider 1,2,5 and Inflation to be mutually exclusive cards, precisely because they don’t make mathematical sense together. Since the Inflation rule changes all numerals (by adding 1) and One, Two, Five! also changes numerals (by changing 3s into 5s), these two rules just don’t play nicely.
So when one comes into play, the other should be discarded if already on the table. They should never be in play at the same time.