 Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2022/discrete-mathematics-continuous-mathematics-and-probability/discrete-mathematics/1/a/iv/solution

7^{4}

unrestricted passwords and

6^{4}

passwords containing no A. By the rule of subtraction, the answer is

7^{4} - 6^{4} = 2401 - 1296 = 1105.

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