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Number theory Difficulty 2.0 Junior Find the answer United Kingdom

Gran's first grandchild guessed that Gran was $75$ , the second $78$ and the third $81$ . It turned out that one of them was mistaken by $1$ year, another one by $2$ years and the other by $4$ years. What is Gran's age?

(A) $76$ (B) $77$ (C) $78$ (D) $79$ (E) impossible to determine

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