Thursday, 18 February 2016

mathematics - One-digit products in a row of numbers


The digits from 1 to 9 can be arranged in a row, such that any two neighbouring digits in this row is the product of two one-digit numbers. Arrangement:



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Is it possible to do such an arrangement using hexadecimal digits 1-9 and A-F? Here the row has 15 digits and all numbers are treated as hexadecimal numbers. Example: 123456789ABCDEF 12=2*9, 23=5*7, 34=4*D, 45 does not work, etc.



Answer



One solution is




D2379A5B6C4E18F

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Thought process:



No product starts with F, so F must be at the end, and the only options are 3F and 8F.
The only 2-digit numbers that are products of 1-digit number and start with digits A,B,C,D,E are A5,A8,A9,B4,B6,C3,C4,D2,E1.


Therefore we must have subsequences E1 and D2. From this, you quickly get an answer by looking at the above table. I don't know if I was lucky, but apart from the observations above, I guessed all the rest right. Just for the sake of it, here is another one: D24E1879A5B6C3F



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