diff --git a/project_euler/problem_001/sol8.py b/project_euler/problem_001/sol8.py
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+"""
+Project Euler Problem 1: https://projecteuler.net/problem=1
+
+Multiples of 3 and 5
+
+If we list all the natural numbers below 10 that are multiples of 3 or 5,
+we get 3, 5, 6 and 9. The sum of these multiples is 23.
+
+Find the sum of all the multiples of 3 or 5 below 1000.
+"""
+
+
+def solution(limit: int = 1000) -> int:
+    """
+    Returns the sum of all the multiples of 3 or 5 below n.
+
+    >>> solution(3)
+    0
+    >>> solution(4)
+    3
+    >>> solution(10)
+    23
+    >>> solution(600)
+    83700
+    >>> solution()
+    233168
+    """
+
+    multiples_of_3 = set(range(3, limit, 3))
+    multiples_of_5 = set(range(5, limit, 5))
+    # Union of two sets — only unique values are preserved
+    result = sum(multiples_of_3 | multiples_of_5)
+
+    return result
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")