Maths Olympiad Prep

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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 EVIL\underline{E}\,\underline{V}\,\underline{I}\,\underline{L} is divisible by 7373, and
- the four-digit number VILE\underline{V}\,\underline{I}\,\underline{L}\,\underline{E} is divisible by 7474.

Compute the four-digit number LIVE\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 VIL=n\underline{V}\,\underline{I}\,\underline{L} = n. Then n2000k(mod73)n \equiv -2000k \pmod{73} and nk/5(mod37)n \equiv -k/5 \pmod{37}, so n1650k(mod2701)n \equiv 1650k \pmod{2701}. We can now exhaustively list the possible cases for kk:
- if k=1k=1, then n1650n \equiv 1650 which is not possible;
- if k=2k=2, then n21650599n \equiv 2 \cdot 1650 \equiv 599, which gives E=4E=4 and n=599n=599;
- if k=3k=3, then n599+16502249n \equiv 599+1650 \equiv 2249 which is not possible;
- if k=4k=4, then n2249+16501198n \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 LIVE=9954\underline{L}\,\underline{I}\,\underline{V}\,\underline{E} = 9954.

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