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wu :: forums    riddles    easy (Moderators: william wu, Grimbal, towr, ThudnBlunder, SMQ, Icarus, Eigenray)    Math riddle? « Previous topic | Next topic » Pages: 1  Reply Notify of replies Send Topic Print    Author  Topic: Math riddle?  (Read 1316 times) x2862 Newbie     Posts: 10 Math riddle?   « on: Jul 19th, 2009, 1:02pm » Quote Modify Can you rearrange the digits of a number n to obtain 2n? Note: n must be a whole number in base 10. I've been trying to find a solution for a long while and found nothing. any help would be nice. IP Logged towr wu::riddles Moderator Uberpuzzler Some people are average, some are just mean.     Gender: Posts: 13730 Re: Math riddle?   « Reply #1 on: Jul 19th, 2009, 2:09pm » Quote Modify on Jul 19th, 2009, 1:02pm, x2862 wrote: Can you rearrange the digits of a number n to obtain 2n? Note: n must be a whole number in base 10. I've been trying to find a solution for a long while and found nothing. any help would be nice. Yes, you can. Consider the decimal expansion of 1/7 IP Logged Wikipedia, Google, Mathworld, Integer sequence DB pex Uberpuzzler     Gender: Posts: 880 Re: Math riddle?   « Reply #2 on: Jul 19th, 2009, 3:18pm » Quote Modify Interestingly, there are twelve possible values for n less than a million, and they all use the same set of digits! IP Logged R Senior Riddler Addicted!!!     Gender: Posts: 502 Re: Math riddle?   « Reply #3 on: Sep 14th, 2009, 12:26am » Quote Modify on Jul 19th, 2009, 2:09pm, towr wrote: Yes, you can. Consider the decimal expansion of 1/7 How did you find this?   @Pex You too!!! IP Logged The first experience seems like Magic, but the second tells you the Trick behind it. pex Uberpuzzler     Gender: Posts: 880 Re: Math riddle?   « Reply #4 on: Sep 14th, 2009, 12:40am » Quote Modify on Sep 14th, 2009, 12:26am, R wrote: How did you find this?   @Pex You too!!! As for me, I simply wrote a bit of code to try all numbers less than a million. towr's solution relies on a well-known property of decimal expansions of fractions: 1/7 = 0.142857142857... 2/7 = 0.285714285714... 3/7 = 0.428571428571... 4/7 = 0.571428571428... 5/7 = 0.714285714285... 6/7 = 0.857142857142... IP Logged towr wu::riddles Moderator Uberpuzzler Some people are average, some are just mean.     Gender: Posts: 13730 Re: Math riddle?   « Reply #5 on: Sep 14th, 2009, 12:55am » Quote Modify on Sep 14th, 2009, 12:26am, R wrote: How did you find this? I remembered it from when it came up in a similar question. IP Logged Wikipedia, Google, Mathworld, Integer sequence DB R Senior Riddler Addicted!!!     Gender: Posts: 502 Re: Math riddle?   « Reply #6 on: Sep 14th, 2009, 5:35am » Quote Modify on Sep 14th, 2009, 12:40am, pex wrote: As for me, I simply wrote a bit of code to try all numbers less than a million.   Ah! Disappointing!!!   Quote: towr's solution relies on a well-known property of decimal expansions of fractions: 1/7 = 0.142857142857... 2/7 = 0.285714285714... 3/7 = 0.428571428571... 4/7 = 0.571428571428... 5/7 = 0.714285714285... 6/7 = 0.857142857142... Can you elaborate on  that property or give link to some good source?   on Sep 14th, 2009, 12:55am, towr wrote: I remembered it from when it came up in a similar question. Well I was going to add "Or you knew this?" and just didn't. I thought, YOU must be having good logical approach anyway. « Last Edit: Sep 14th, 2009, 5:43am by R » IP Logged The first experience seems like Magic, but the second tells you the Trick behind it. towr wu::riddles Moderator Uberpuzzler Some people are average, some are just mean.     Gender: Posts: 13730 Re: Math riddle?   « Reply #7 on: Sep 14th, 2009, 6:29am » Quote Modify on Sep 14th, 2009, 5:35am, R wrote: Well I was going to add "Or you knew this?" and just didn't. I thought, YOU must be having good logical approach anyway. Nah, if I hadn't remember this, I'd have used pex's approach and run a program. IP Logged Wikipedia, Google, Mathworld, Integer sequence DB pex Uberpuzzler     Gender: Posts: 880 Re: Math riddle?   « Reply #8 on: Sep 14th, 2009, 9:08am » Quote Modify on Sep 14th, 2009, 5:35am, R wrote: Can you elaborate on  that property or give link to some good source? MathWorld has a page on it. IP Logged victorpride Newbie     Posts: 4 Re: Math riddle?   « Reply #9 on: Feb 10th, 2010, 8:27pm » Quote Modify umm the question specifically stated it must be a whole number. IP Logged ThudnBlunder wu::riddles Moderator Uberpuzzler The dewdrop slides into the shining Sea     Gender: Posts: 4489 Re: Math riddle?   « Reply #10 on: Feb 10th, 2010, 9:47pm » Quote Modify on Feb 10th, 2010, 8:27pm, victorpride wrote: umm the question specifically stated it must be a whole number. 142857 x 2 = 285714 IP Logged THE MEEK SHALL INHERIT THE EARTH.....................................................................er, if that's all right with the rest of you. Pages: 1  Reply Notify of replies Send Topic Print Forum Jump: ----------------------------- riddles -----------------------------=> easy   - medium   - hard   - what am i   - what happened   - microsoft   - cs   - putnam exam (pure math)   - suggestions, help, and FAQ   - general problem-solving / chatting / whatever ----------------------------- general -----------------------------  - guestbook   - truth   - complex analysis   - wanted   - psychology   - chinese « Previous topic | Next topic »

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