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Not all a's are in (Posted on 2018-02-06) Difficulty: 2 of 5
A four-digit PIN number aabb, where a and b are distinct digits, is a multiple of 15.

Which values of a are not possible?

See The Solution Submitted by Ady TZIDON    
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re: Solution (spoil alert) | Comment 2 of 3 |
(In reply to Solution (spoil alert) by JayDeeKay)

So the possible values of a are when:

aa00 is divisible by 3, => 2a is mod3  => a=3, 6, 9
Then aabb = (3300, 6600, 9900)


aa55 is divisible by 3, => 2a+1 is mod 3 => a=1, 4, 7
Then aabb = (1155, 4455, 7755)

0 and 5 are values of b so they can't be values of a

Then the values of a that are not possible (to add 0 and 5) are only 2 and 8.  

  Posted by armando on 2018-02-06 14:55:40
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