I keep getting “Inclusion–exclusion” questions wrong in Sets & Statistics. What's the trap?
adding the two group sizes and forgetting to remove the overlap.
The usual trap: adding the two group sizes and forgetting to remove the overlap. How to avoid it: subtract the 'both' group once to avoid double counting. Go back to the definition — for two sets, n(A∪B) = n(A) + n(B) − n(A∩B) — you subtract the overlap so it isn't double-counted.
Key point
Inclusion–exclusion
Common mistake
adding the two group sizes and forgetting to remove the overlap.
Memory tip
subtract the 'both' group once to avoid double counting.
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