How many four-digit numbers can be formed with the digits 1, 2, 3, 4, 5, 6 when a digit may be repeated any number of times in any arrangement ?

Explanation

To solve the problem of how many four-digit numbers can be formed using the digits 1, 2, 3, 4, 5, and 6 with repetition allowed, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the total number of digits available**:
We have the digits 1, 2, 3, 4, 5, and 6. This gives us a total of 6 different digits.

2. **Determine the number of positions in the four-digit number**:
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