IBM Research | Ponder This | September 2008 solutions
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September 2008

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The minimum number is 11 as illustrated below. This problem is from Sam Loyd via Martin Gardner.


1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111AAAAAA
1111111244455
BBBBBB3344455
BBBBBB3344466
BBBBBB7777866
BBBBBB7777999
BBBBBB7777999
BBBBBB7777999


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