how many two digit number can be formed with these numbers 1,2,3,4. (repetition is allowed)

The answer is 16.

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Since repetition is allowed, then: 11, 12, 13, and 14; 21, 22, 23, and 24; 31, 32, 33, and 34; 41, 42, 43, and 44.

Total: 16 different 2-digit numbers