Password Permutations

What this combinatorics interview question tests

This is an easy counting problem that assesses your ability to apply the multiplication principle and handle the constraint of uniqueness (no repetition). It appears frequently in probability and statistics rounds because it requires clear thinking about ordered vs. unordered selections and conditional probability in a low-stakes setting.

To solve problems like this, you identify how many choices you have at each step, then multiply them together. The key insight is recognizing that after you make one choice, the number of available options for the next position changes. Working through the problem systematically—handling letters first, then digits—and being careful about the constraint that characters must be unique will lead to the answer.

• Permutations vs. combinations
• The multiplication principle for counting
• Effects of the "without replacement" constraint on successive choices