GeekDad Puzzle of the Week: Finger Primes

Geek Culture

Fifteen, as represented in Nora’s counting scheme.

The other evening at dinner, Max and Nora were counting out numbers on their fingers. Max, age five (almost six!) was using both hands to count to ten — making base 10 numbers. Nora, on the other hand, just turned three, and was counting in a less traditional method. She counted from one to ten, and then made “two hand” two-digit numbers, counting ten (1-0) to fifteen (1-5), and then jumped to twenty (2-0) through twenty five (2-5.) She was stumped at fifty-five (5-5), and then borrowed one of my fingers to start again at one-hundred (1-0-0.)

With the eight hands at the table, as my dear wife was with us of course, we could count in this manner up to 55,555,555, the largest 8-digit number represented upon fingers. This week’s GeekDad Puzzle of the Week is once again straightforward — if we did indeed count from zero (00,000,000) to this MaxInt (pun intended) of 55,555,555 with no single digit larger than a hand’s count of digits (five), how many primes would we encounter? Note that for purposes of this puzzle, any given number’s divisors need not be representable on fingers. That is, 35 (a number we would encounter) is still composite despite us not being able to represent one of its divisors, 7. Standard base 10 multiplication and division rules still apply.

For your chance at this week’s $50 ThinkGeek Gift Certificate, please submit your response by end of day Friday to GeekDad Central. Please use any “digital” means available to calculate your answer. Good luck!

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