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UKMT Intermediate Mathematical Challenge, 2017

Number theory Difficulty 1.0 Junior Find the answer United Kingdom

The number 'ab' is the two-digit number with units digit $b$ and tens digit $a$ . The digits $a$ and $b$ are distinct, and non-zero. What is the largest possible value of 'ab' $-$ 'ba'?

(A) $81$ (B) $72$ (C) $63$ (D) $54$ (E) $45$

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