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Permutations and Remainders (Posted on 2024-05-17) Difficulty: 3 of 5
Determine all the numbers formed by three different and non-zero digits, such that the six numbers obtained by permuting these digits leaves the same remainder after the division by 4.

No Solution Yet Submitted by Danish Ahmed Khan    
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Solution Analytic solution with program confirmation | Comment 3 of 4 |
All 3 digits must have the same parity, so the odd digits can be analyzed separately from the even digits.

The mod4 value of a 3-digit number is the same as the mod4 value of the last 2 digits.  

Evens: Consider flipping the last 2 digits.  What pairs of even digits have the same mod4 value when reversed?  26, 48 (or 04, but zero is excluded).  The 2 digits must differ by 4, so the only triplet would be {0,4,8} but zero is excluded.  So there is no solution with even digits.

Odds:  if both digits of a 2-digit number are odd, which ones do not have their mod4 value changed by reversing the digits?  15, 19, 37, 59  Again, the difference must be 4 (or a multiple of 4).
The only triplet which can be formed from these pairs is:  {1,5,9}.

Confirmed with a short program.
('1', '5', '9')
159 3
195 3
519 3
591 3
915 3
951 3

------------
from itertools import combinations
from itertools import permutations

for comb in combinations('123456789', 3):
    remainder = int(''.join(comb)) % 4
    winner = True
    for perm in permutations(comb):
        x = int(''.join(perm))
        if x % 4 != remainder:
            winner = False
            break
    if winner:
        print(comb)
        for perm in permutations(comb):
            x = int(''.join(perm))
            print(x, x%4)

  Posted by Larry on 2024-05-17 13:23:55
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