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UKMT Intermediate Mathematical Challenge, 2018

Algebra Difficulty 1.0 Junior Find the answer United Kingdom

Someone has switched the numbers around on Harry's calculator! The numbers should be in the positions shown in the left-hand diagram, but have been switched to the positions in the right-hand diagram. Which of the following calculations will not give the correct answer when Harry uses his calculator?

(A) $79 \times 97$ (B) $78 \times 98$ (C) $147 \times 369$ (D) $123 \times 321$ (E) $159 \times 951$

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