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Number theory Difficulty 7.1 National olympiad, round 2 Prove it Argentina

By writing the digits 11, 22, 33, 44, 55, 66, 77, 88 and 99 in the cells of a 3×33 \times 3 board, without repetitions, 66 numbers of 33 digits each are formed: one in each row and one in each column. For instance, if the board is filled in like in this picture

Column 1Column 2Column 3
Row 1127
Row 2563
Row 3498

then the 66 numbers are: 127127, 563563, 498498, 154154, 269269 and 738738.

We have to fill in the 3×33 \times 3 board so that the number in the first row is a multiple of 22, the number in the second row is a multiple of 33, the number in the third row is a multiple of 44, the number in the first column is a multiple of 55, the number in the second column is a multiple of 66, and the number in the third column is a multiple of 77.

Determine all possible ways to fill in the board.

Solution

In order for the number in the first column to be a multiple of 55, we must write 55 in the cell corresponding to its units, that is, in row 33 and column 11. The digits in the cells corresponding to the units of the numbers that are multiple of 22, 44 and 66 must be even. Then, the number in the third row (which is a multiple of 44) begins with 55 and its remaining two digits are even. The multiples of 44 satisfying this condition, with no repeated digits and without 00, are
524, 528, 548, 564, 568, 584. 524,\ 528,\ 548,\ 564,\ 568,\ 584.
In particular, we deduce that the digit in row 33, column 33 can only be 44 or 88.

even
5even4 - 8

For the number in the third column, we look for the multiples of 77 beginning with an even digit, ending with 44 or 88, with no repeated digits, and without 00 and 55 among its digits. There are two: 238238 and 294294. Combining them with the numbers listed above, that are the candidates for the third row (note that 524524 and 528528 cannot be used, since 22 must be written in the first row), we obtain the following possibilities:
2
3
548

Board 1
2
3
568

Board 2
2
9
564

Board 3
2
9
584

Board 4

To determine the values of the remaining digits, we take into account that the sums of the digits in row 22 and the sum of the digits in column 22 must be multiples of 33 (in order that the corresponding numbers are multiple of 33 and 66, respectively).

Board 1: The remaining digits are 11, 66, 77, 99; thus, two are congruent with 11 modulo 33 and two are congruent with 00 modulo 33. We write the board modulo 33:

ab2
cd0
212

If d=1d = 1, the number in the second row cannot be a multiple of 33 and, if d=0d = 0, the number in the second column cannot be a multiple of 33. Then, there is no solution for Board 1.

Board 2: The remaining digits are 11, 44, 77, 99; one is congruent with 00 modulo 33 and three are congruent with 11 modulo 33. As before, by writing the board modulo 33

ab2
cd0
202

and considering both possible values of dd, it follows that it is not possible to fill in the board satisfying the required conditions.

Board 3: The remaining digits are 11, 33, 77, 88; two are congruent with 11 modulo 33, one is congruent with 00 modulo 33 and the remaining one is congruent with 22 modulo 33. There is a unique way to fill in the board modulo 33:

012
120
201

which leads to the following two solutions:
312
789
564
372
189
564

Board 4: The remaining digits are 11, 33, 66, 77; two are congruent with 00 modulo 33, and two are congruent with 11 modulo 33. There is a unique way to fill in the board modulo 33:
112
000
221

leading to the following solutions:
172
369
584
712
369
584
172
639
584
712
639
584

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.