How many numbers of four digits can be formed with the digits 1, 2, 3, 4, 5 if the digits can be repeated in the same number?
The thousand's place can be filled by any of the 5 digits.
∴ Number of ways of filling the thousand's place = 5
Since the digits can repeat in the number, the hundred's place, the ten's place and the unit's place can each be filled in 5 ways.
$\therefore$ Total numbers $=5 \times 5 \times 5 \times 5=625$
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