232.Products Ending in 1, 3, 7, or 9

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classical probabilityCh. 1 — Classical Probability and Combinatorics444+ Problems in Probability

Four numbers from

\{1, 2, \dots, 9\}

are selected with repetition allowed. Find the probability that the product of the four integers has a units digit of

1

3

7

, or

9

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