Practice question · Numerical answer
Events A and B satisfy P(A) = 0.5, P(B) = 0.4 and P(A and B) = 0.2. Compute P(A or B).
Hints
- These events overlap, so simple addition double-counts something.
- Add the two probabilities, then subtract the intersection once.
Show the answer
0.7 (answers within ±0.001 count)
Why
P(A or B) = 0.5 + 0.4 - 0.2 = 0.7. Adding without subtracting would give 0.9, counting the shared outcomes twice. This is inclusion-exclusion, the same principle already met with sets.
Practise Introduction to Probability
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