When the direct count is a headache, count the opposite instead, so "at least one" becomes 1 minus "none at all," P(A) = 1 − P(Ā)
Module goal. Treat events like sets you can add and subtract, building the addition formula and the language of mutually exclusive and complementary events
eastmath.com · 33.3 Operations and Relationships Between Events · 33.3.4 Finding Probabilities via the Complement