Logic / Unit 7 Logic 347 short lesson, about 20 seconds
Disjunction Rules
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The idea
Mutually exclusive: P(A or B) = P(A) + P(B). Otherwise: add, then subtract P(A and B).
In practice
One die, "2 or 5": add. "King or heart": add, subtract the king of hearts — counted twice.
Where people slip
The subtraction exists because inclusive-or double-counts the overlap. Video 120's distinction, now with arithmetic.
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