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Why is it 7 in the denominator?

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It is given "3 appears twice as often as each other number". We can say from this statement

{1},{2},{4},{5},{6} are equally likely events therefore P(1) = P(2) = P(4) =P(5) =P(6) = (say,x)

while probability of event {3} = 2x

Sum of all probabilities = 1 = 2x + 5x

x = 1/7 Therefore P(1) = P(2) = P(4) =P(5) =P(6) = 1/7  while P(3) = 2/7

Probability that an odd number appears = P(1) + P(3) + P(5) = 1/7 + 2/7 + 1/7 = 4/7

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