What is the last digit of: × × 2 2 1 2 1 2 … × 3 3 × 1 3 1 3 × 4 4 × 1 4 1 4 × 6 6 × 1 6 1 6 × 7 7 × 1 7 1 7 × 8 8 × 1 8 1 8 × 9 9 × 1 9 1 9 × 9 9 9 9
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Hm, can you review this solution? I'm guessing that you didn't see the full extent of the problem (which I have now edited for clarity).
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Oops...it looked like the product was ending at 1 7 1 7 . Actually, since ( n + 2 0 ) n + 2 0 ≡ n n ( m o d 1 0 ) , I was not that far off. I was just "lucky" that 1 8 1 8 ∗ 1 9 1 9 ≡ 1 ( m o d 5 ) .
the last digit will be modeled by the last digit of 2 n 2 × 3 n 3 × 4 n 4 × 6 n 6 × 7 n 7 × 8 n 8 × 9 n 9 where n 3 and n 7 are divisible by 4, and n 2 and n 8 are even but not divisible by 4, and n 4 and n 9 are even, thus the last digit of the above product reduces to 4 × 1 × 6 × 6 × 1 × 4 × 1 = 6 . The cyclical patterns of the last digit of the powers of each digit are as follows: 2 − > ( 2 , 4 , 8 , 6 ) , 3 − > ( 3 , 9 , 7 , 1 ) , 4 − > ( 4 , 6 ) , 5 − > ( 5 ) , 6 − > ( 6 ) , 7 − > ( 7 , 9 , 3 , 1 ) , 8 − > ( 8 , 4 , 2 , 6 ) , 9 − > ( 9 , 1 )
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Working modulo 5 (and modulo ϕ ( 5 ) = 4 in the exponents), the given number n reduces to 2 2 ∗ 3 3 ∗ 2 3 ∗ 4 ∗ 3 ∗ 4 2 ∗ 2 = 2 1 2 ∗ 3 4 ≡ 1 ( m o d 5 ) . Since n is even, the last digit must be 6