I'm having a difficult time determining if a certain math problem should be classified as using Formal or Informal Logic. Here it is: 1. ALL except 2 of my pets are dogs. 2. ALL except 2 of my pets are cats. 3. ALL except 2 of my pets are birds. Q: How many pets do I own? A: 2 or 3 So, while it's obvious why the answer could be 3, it's not obvious how it could be 2 as well. The reason why is because the phrase "All" could be zero, which would represent an empty set. And, of course, I could own pets other than the ones mentioned (fish / lizards). So, knowing that, we can substitute that example back into the original problem as follows: I own two, pets, which are both fish. All except 2 of my pets are dogs, which in this case, is equal to zero. So, the set of dogs can possibly be an empty set. So, anyways, I was wanting to know if the puzzle itself could be considered "formal", or is it informal because most people would mean "All" to at least equal one, and we add that assumption in there?