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The only case in which machine will not work : Both A and B fail

Probability that machine will work = 1 - Probability of both A and B failing

Probability of A failing = 1 - 2/3 = 1/3

Probability of B failing = 1 - 2/3 = 1/3

Probability of both A and B failing = (1/3)*(1/3) = 1/9

Probability that machine will work = 1 - 1/9 = 8/9
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