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Baltic Way 1993 · Problem 1

Number Theory

a1a2a3‾\overline{a_{1} a_{2} a_{3}} and a3a2a1‾\overline{a_{3} a_{2} a_{1}} are two three-digit decimal numbers, with a1,a3a_{1}, a_{3} being different non-zero digits. The squares of these numbers are five-digit numbers b1b2b3b4b5‾\overline{b_{1} b_{2} b_{3} b_{4} b_{5}} and b5b4b3b2b1‾\overline{b_{5} b_{4} b_{3} b_{2} b_{1}} respectively. Find all such threedigit numbers.

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Topics

Divisibility and factorization

Solutions

Solution

Solution:

Assume a1>a3>0a_{1} > a_{3} > 0. As the square of a1a2a3‾\overline{a_{1} a_{2} a_{3}} must be a five-digit number we have a1≤3a_{1} \leq 3. Now a straightforward case study shows that a1a2a3‾\overline{a_{1} a_{2} a_{3}} can be 301, 311, 201, 211 or 221.

Contest context

Results from Baltic Way 1993

8 teams

Mean score
4.4 / 5
Scores of 4 or 5
7 / 8
Estonia
5 / 5

Score distribution

00
10
20
31
43
54
All team scores
TeamScore
Poland5 / 5
Latvia4 / 5
Estonia5 / 5
Sweden4 / 5
Lithuania5 / 5
Finland3 / 5
Iceland5 / 5
Denmark4 / 5