67

This number is a prime.

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John Pilkins ©2003

Any digit n, 1 through 9, times 67 yields a 3-digit number (for the purpose of this discussion make 1*67 = 067), then the concatenation of the first and last digits of the product equals n times 7. E.g., 8*67 = 536 and 8*7 = 56. [Green]