Question

Find the three-digit positive integer $\underline{a}\,\underline{b}\,\underline{c}$ whose representation in base nine is $\underline{b}\,\underline{c}\,\underline{a}_{\,\text{nine}},$ where $a,$ $b,$ and $c$ are (not necessarily distinct) digits.

答案

Solution:

The condition says $100a + 10b + c = 81b + 9c + a,$ which simplifies to $99a = 71b + 8c.$ Since the digits also appear in a base-nine numeral, each is at most $8.$ Reducing modulo $8$ gives $3a \equiv -b \pmod 8,$ so $b \equiv 5a \pmod 8.$

For $a = 1,$ $b = 5$ makes $71b$ exceed $99;$ for $a = 2,$ $b = 2$ gives $8c = 198 - 142 = 56,$ so $c = 7.$ For each $a \ge 3,$ the required $b$ forces $99a - 71b$ outside the range $[0, 64],$ so there is no other solution.

The number is $227,$ and indeed $227 = 2 \cdot 81 + 7 \cdot 9 + 2 = 272_{\text{nine}}.$

2022 AIME I Problem 2

Solution:

Problem 2 in Other Years