Imagine a rectangle divided into 3x4 squares, and put a digit in each square.

     +---+---+---+---+
     | a | b | c | d |  A
     +---+---+---+---+
     | e | f | g | h |  B
     +---+---+---+---+
     | i | j | k | l |  C
     +---+---+---+---+
       D   E   F   G

The number abcd is denoted by A, that is, A = 1000a + 100b + 10c + d, and the same for the other 2 horizontal numbers B and C.

The number aei is denoted by D, that is, D = 100a + 10e + i, and the same for the other 3 vertical numbers E, F and G.

Prove that if any 6 of these numbers (A, B, C, D, E, F, G) are divisible by 7, then the last number must also be divisible by 7.

(In reply to Proof and Extensions by Steve Herman)

"I expect that this will work for any number of rows and columns, and any modulus, and any base. ".

1234
5678
9024

Modulus 2: All three horizontal numbers (still base 10) are even. Only three of the vertical numbers are even. One is odd.

Posted by Charlie on 2008-11-06 11:47:26