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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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Solution (spoil alert) | Comment 1 of 3
15 = 5 * 3
Therefore, all PIN candidates are multiples of both 3 and 5.
Multiples of 5 end in 0 or 5 ==> b = 0 or 5

To find the "possible" values of a, we need only find numbers aa00 and aa55 which are evenly divisible by 3. The rest are "not possible."

A quick scan for "not possible values" of a finds:
      a = 2, 3, 5, 6, 8, 9  when b = 5
and a = 1, 2, 4, 5, 7, 8  when b = 0


  Posted by JayDeeKay on 2018-02-06 12:27:58
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