Maths Olympiad Prep

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HMMT February

Number theory Difficulty 5.1 AIME, harder Find the answer United States

Problem:

Suppose E,I,L,VE, I, L, V are (not necessarily distinct) nonzero digits in base ten for which
- the four-digit number E‾ V‾ I‾ L‾\underline{E}\,\underline{V}\,\underline{I}\,\underline{L} is divisible by 7373, and
- the four-digit number V‾ I‾ L‾ E‾\underline{V}\,\underline{I}\,\underline{L}\,\underline{E} is divisible by 7474.

Compute the four-digit number L‾ I‾ V‾ E‾\underline{L}\,\underline{I}\,\underline{V}\,\underline{E}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

Let E‾=2k\underline{E} = 2k and V‾ I‾ L‾=n\underline{V}\,\underline{I}\,\underline{L} = n. Then n≡−2000k(mod73)n \equiv -2000k \pmod{73} and n≡−k/5(mod37)n \equiv -k/5 \pmod{37}, so n≡1650k(mod2701)n \equiv 1650k \pmod{2701}. We can now exhaustively list the possible cases for kk:
- if k=1k=1, then n≡1650n \equiv 1650 which is not possible;
- if k=2k=2, then n≡2⋅1650≡599n \equiv 2 \cdot 1650 \equiv 599, which gives E=4E=4 and n=599n=599;
- if k=3k=3, then n≡599+1650≡2249n \equiv 599+1650 \equiv 2249 which is not possible;
- if k=4k=4, then n≡2249+1650≡1198n \equiv 2249+1650 \equiv 1198 which is not possible.

Hence, we must have (E,V,I,L)=(4,5,9,9)(E, V, I, L) = (4, 5, 9, 9), so L‾ I‾ V‾ E‾=9954\underline{L}\,\underline{I}\,\underline{V}\,\underline{E} = 9954.

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