Solution:Each letter can be put into any one of three mailboxes. Since there are
four letters, the number of ways that the four letters can be put into three mailboxes is 3 x 3 x 3 x 3 = 81 (By the counting principle) |
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| Problem 1 | Problem 2 | Problem 3 | Problem 4 | Problem 5 |

| Problem 6 | Problem 7 | Problem 8 | Problem 9 | Problem 10 |