BMAD 64 Posted February 22, 2018 Report Share Posted February 22, 2018 Say 5939 is a "right" prime because it remains prime after dropping any number of digits from the right: 5939, 593, 59, and 5 are all prime. How many right primes are there less than 1000? Quote Link to post Share on other sites

0 Solution Molly Mae 103 Posted February 23, 2018 Solution Report Share Posted February 23, 2018 (edited) Some amount of primes. 27 2, 3, 5, 7, 23, 29, 31, 37, 53, 59, 71, 73, 79, 233, 239, 293, 311, 313, 317, 373, 379, 593, 599, 719, 733, 739, 797 So I started with single digit prime numbers, then I just built a tree for two-digit primes that begin with one of those one-digit primes. I did the same thing for the three-digit primes that begin with one of the two-digit primes. Edited February 23, 2018 by Molly Mae Quote Link to post Share on other sites

1 Pickett 13 Posted February 27, 2018 Report Share Posted February 27, 2018 (edited) FWIW Spoiler There are only 83 total...and they are all under 100,000,000... Spoiler Also going with the "tree" approach that Molly Mae mentioned: 2 23 233 2333 23333 23339 2339 23399 233993 2339933 23399339 239 2393 2399 23993 239933 2399333 29 293 2939 29399 293999 2939999 29399999 3 31 311 3119 31193 313 3137 31379 317 37 373 3733 37337 373379 3733799 37337999 37339 373393 3739 37397 379 3793 3797 5 53 59 593 5939 59393 593933 5939333 59393339 59399 593993 599 7 71 719 7193 71933 719333 73 733 7331 7333 73331 739 7393 73939 739391 7393913 73939133 739393 7393931 7393933 739397 739399 79 797 Edited February 27, 2018 by Pickett typos. Quote Link to post Share on other sites

0 SUCHIT BEHAL 0 Posted February 23, 2018 Report Share Posted February 23, 2018 Any other prime numbers less than 1000. Quote Link to post Share on other sites

0 rocdocmac 9 Posted February 23, 2018 Report Share Posted February 23, 2018 Spoiler 23 "right" primes if 2, 3, 5 and 7 (nothing to drop on the right) are not considered. Quote Link to post Share on other sites

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## BMAD 64

Say 5939 is a "right" prime because it remains prime after dropping any number of digits from the right: 5939, 593, 59, and 5 are all prime. How many right primes are there less than 1000?

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