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Suppose we have four integers, no two of which are congruent $\pmod 6$. Let $N$ be the product of the four integers. If $N$ is not a multiple of $6$, then what is the remainder of $N$ when $N$ is divided by $6$?

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mathmate

Answer:

4

Step-by-step explanation:

Given:

A, B, C, D have distinct positive values for mod 6

A (mod 6) = 1

B (mod 6) = 2

C (mod 6) = 4

D (mod 6) = 5

Each mod 6 value cannot be a zero since the product ABCD is not a multiple of 6.

Furthermore, in order that ABCD mod 6 > 0, we cannot  have a residue equal to 3, else the product with a residue 2 or 4 will make the product a multiple of 6.

Thus the only positive residues can only be 1,2,4,5

A*B*C*D  (mod 6) > 0  = 1*2*4*5 (mod 6) = 4

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