A fair die is rolled 10 independent times, (the 10 rolls are independent). Let X denote the number of outcomes that appear exactly twice. For example, if we roll (4, 6, 5, 6, 3, 1, 2, 2, 6, 4), then 4 and 2 each appear twice, 6 appears 3 times, and 1, 3, and 5 appear once each. In this case X=2. Find the mean and variance of X.

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