Answer: P(F) = 0.35
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Work Shown:
P(E or F) = P(E) + P(F) - P(E and F)
0.55 = 0.40 + P(F) - 0.20
0.55 = P(F) + 0.20
P(F) + 0.20 = 0.55
P(F) = 0.55 - 0.20
P(F) = 0.35
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note: Sometimes you may see the formula as
P(E and F) = P(E) + P(F) - P(E or F)
but it's an equivalent form to the first equation above.