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Algebra Difficulty 4.5 AIME Prove it Estonia

Find all four-digit numbers which are exactly 20162016 larger than the four-digit number obtained by moving the first digit to the end.

Solution

Let the first digit of the number be aa and the number formed by the remaining digits be kk. By the conditions, 1000a+k=10k+a+20161000a + k = 10k + a + 2016, whence 111ak=224111a - k = 224. Hence a3a \ge 3, implying the solutions a=3,k=109a = 3, k = 109; a=4,k=220a = 4, k = 220; a=5,k=331a = 5, k = 331; a=6,k=442a = 6, k = 442; a=7,k=553a = 7, k = 553; a=8,k=664a = 8, k = 664; a=9,k=775a = 9, k = 775. The corresponding four-digit numbers satisfying the conditions of the problem are 31093109, 42204220, 53315331, 64426442, 75537553, 86648664, and 97759775.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.