Take S to the the set of all voters eligible to vote in the last two elections. The events we are interested in are
E: a voter voted in the first; P(E) = .25
F: a voter voted in the second; P(F) = x (this is the unknown)
EF: a voter voted in both; P(EF) = .20
EF: a voter voted in at least one election; P(EF) = 1 - .35 = .65
(since 35% voted in neither, the remaining 65% voted in at least one election).
Putting all this together,
P(EF) = P(E) + P(F) - P(EF)
0.65 = 0.25 + x - .20,
giving x = .60, or 60%
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