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wu :: forums    riddles    easy (Moderators: ThudnBlunder, towr, Eigenray, Icarus, SMQ, Grimbal, william wu)    make the sum zero « Previous topic | Next topic » Pages: 1  Reply Notify of replies Send Topic Print    Author  Topic: make the sum zero  (Read 878 times) sak_lko Newbie     Posts: 4 make the sum zero   « on: Aug 2nd, 2010, 11:34am » Quote Modify You are given digits 1 to 30 i.e. 1..2..3..4......30. Use only + and - signs , and use all the digits only once..so as to make the sum zero   For ex: for digits 1 to 3: -1-2+3=0 IP Logged sak_lko Newbie     Posts: 4 Re: make the sum zero   « Reply #1 on: Aug 2nd, 2010, 11:57am » Quote Modify Anyone?? IP Logged towr wu::riddles Moderator Uberpuzzler Some people are average, some are just mean.     Gender: Posts: 13730 Re: make the sum zero   « Reply #2 on: Aug 2nd, 2010, 12:02pm » Quote Modify Begin by adding all numbers Then notice it doesn't divide nicely by 2; so you can't find two equal subsets   [edit]note to self, stop confusing can and can't[/edit] « Last Edit: Aug 2nd, 2010, 12:51pm by towr » IP Logged Wikipedia, Google, Mathworld, Integer sequence DB sak_lko Newbie     Posts: 4 Re: make the sum zero   « Reply #3 on: Aug 2nd, 2010, 12:10pm » Quote Modify the sum comes to 465...tried to find the subsets..still stumped « Last Edit: Aug 2nd, 2010, 12:10pm by sak_lko » IP Logged towr wu::riddles Moderator Uberpuzzler Some people are average, some are just mean.     Gender: Posts: 13730 Re: make the sum zero   « Reply #4 on: Aug 2nd, 2010, 12:21pm » Quote Modify Suppose you divide the numbers into two sets, the ones you add go into set A, and the ones you subtract form set S. Then sum(A) + sum(S) = 465, right? Now, you want sum(A) - sum(S) to be 0. But that would mean sum(A) = sum(S) = 465/2, which is impossible, since both have to be whole numbers. So in short, there is no way to do it. « Last Edit: Aug 2nd, 2010, 12:22pm by towr » IP Logged Wikipedia, Google, Mathworld, Integer sequence DB sak_lko Newbie     Posts: 4 Re: make the sum zero   « Reply #5 on: Aug 2nd, 2010, 12:35pm » Quote Modify thnx...  i guess thats why it took so long,,finally i can go to bed...   IP Logged 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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