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<p>[QUOTE="Numbers, post: 434197, member: 11668"]Easy: the first half of the digits can be anything at all, and the second half then have to be an exact match (in reverse). So just count the number of different combinations that are possible for the first half of the digits.</p><p><br /></p><p>For six-digit serials, that makes 1000 radars out of every block of 1,000,000 notes, so you get a radar once every 1000 notes. (Make small adjustments if e.g. serial number 000000 isn't printed; such things won't affect the final answer by more than a hair.)</p><p><br /></p><p>For eight-digit serials, there are 10,000 radars out of every block of 100,000,000 notes, so radars are printed once every 10,000 notes.</p><p><br /></p><p>And for the benefit of our Canadian friends: For seven-digit serials, there are 10,000 radars out of every block of 10,000,000 notes, so the radars come up once every 1000 notes. It's interesting to observe that radars are equally common for six-digit and seven-digit serial formats....</p><p><br /></p><p>Any more math problems out there? <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie6" alt=":cool:" unselectable="on" unselectable="on" />[/QUOTE]</p><p><br /></p>
[QUOTE="Numbers, post: 434197, member: 11668"]Easy: the first half of the digits can be anything at all, and the second half then have to be an exact match (in reverse). So just count the number of different combinations that are possible for the first half of the digits. For six-digit serials, that makes 1000 radars out of every block of 1,000,000 notes, so you get a radar once every 1000 notes. (Make small adjustments if e.g. serial number 000000 isn't printed; such things won't affect the final answer by more than a hair.) For eight-digit serials, there are 10,000 radars out of every block of 100,000,000 notes, so radars are printed once every 10,000 notes. And for the benefit of our Canadian friends: For seven-digit serials, there are 10,000 radars out of every block of 10,000,000 notes, so the radars come up once every 1000 notes. It's interesting to observe that radars are equally common for six-digit and seven-digit serial formats.... Any more math problems out there? :cool:[/QUOTE]
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