A false coin

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</NOSCRIPT></OBJECT><!-- EndAftershock mm/Coins.swf --> <p> <script type="text/javascript"><!-- google_ad_client = "pub-7892672890047209"; google_ad_width = 468; google_ad_height = 60; google_ad_format = "468x60_as"; google_ad_type = "text"; google_ad_channel =""; google_color_border = "FFFFFF"; google_color_bg = "FFFFFF"; google_color_link = "0000FF"; google_color_url = "008000"; google_color_text = "000000"; //--></script> <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script> <h3>Explanation in text</h3> You make three groups of three coins...<br><br> You weigh two groups. If the group of coins are equal in weight, it must be the third group, if one group is lighter the false (and lighter) coin must be in that group.<br><br><br> Now you'll continue with the remaining group of three coins and devide them in three...<br><br> You weigh two coins. If they are equal in weight, it must be the third coin, if one coin is lighter is must be that coin.<br><br><br> <h3>Advanced follow up</h3> <A HREF="mailto:harveykatz@gnn.com">Harvey Katz</A> has sent a fantastic follow-up, making it some more complex ! <p><b>Question:</b> <br>There are 12 coins, 11 of which weigh the same. Find the odd coin with 3 uses of a balance. <p><b>Answer:</b> <br>Weigh 4 coins (#'s 1,2,3,4 against 4 others (#'s 5,6,7,8). <p>1. If equal: Weigh #'s 1,2,3 against 9,10,11. <br>a. If equal weigh #1 against # 12 --- done. <br>b. If 1,2,3 is light, weigh 9 against 10 - heavy one is it - if equal #12 is it (heavy). <br>c. If 1,2,3 is heavy - same as b. but light one is it - if equal #12 is it (light). <p>2. If 1,2,3,4 is light: <br>- Designate 1,2,3,4 as all "L" (light if bad one is here) and 5,6,7,8 as "H" (heavy if bad one is here). <br>- Weigh 1,2,5 (L,L,H) against 3,4,6 (L,L,H). <br>- If 1,2,5 is light then the bad coin is either 1,2 as light or 6 as heavy. Simply weigh 1 against 2. The rest is obvious for the other case. <br><br> <tr><td bgcolor=444444><center> <a href="http://www.mindbreakers.com/" target="_top"><img border=0 width=550 height=16 src="img/sub_file!.gif" Alt=" "></a></td></tr> </td></tr></table><br>