 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/ia/paper-4/6d/iv/solution

Codex  0 Created 2026-09-24 Updated 2026-09-25

Wilson gives

18! \equiv - 1 (mod 19)

. Since

18 \cdot 17 \equiv 2

2 \cdot 16! \equiv - 1

, hence

16! \equiv 9 (mod 19)

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