How so? The question explicitly asks this: "probability that it rains on fewer than 3 days".That will get you one mark out of two.
Oh never mind, I skimmed and thought your justification was for why 0 was to be included, not why 0 is NOT to be included.
You may or may not get a mark. Leaning towards not.
Zero days means that no rain occurs, hence should not be included in your expression. It's flawed, flawed hard... *inserts formal complaint*