How many four-digit natural numbers are there such that <strong>ALL</strong> of the digits are odd?

Explanation

The odd digits available are 1, 3, 5, 7, 9, giving 5 possible choices. For a four-digit number where every digit is odd, each of the four places can be filled in 5 ways. The total number of ways is 5 \times 5 \times 5 \times 5 = 625.

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