2021 AMC 10A Fall Problem 12

Attempt Problem 12 of the 2021 AMC 10A Fall below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Fall solutions, or check the answer key.

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12.

The base-nine representation of the number NN is 27,006,000,0529.27,006,000,052_9. What is the remainder when NN is divided by 5?5?

00

11

22

33

44

Answer: D
Concepts:number basemodular arithmetic
Difficulty rating: 1070
Solution:

Note that 91(mod5). 9 \equiv -1 \pmod{5}. Then if expand NN using the definition of bases, we get N=2910+799+696+ N = 2 \cdot 9^{10} + 7 \cdot 9^9 + 6 \cdot 9^6 +59+2 5 \cdot 9 + 2 2(1)10+7(1)9+6(1)6+ \equiv 2 (-1)^{10} + 7 (-1)^9 + 6 (-1)^6 +5(1)+2(mod5) 5 (-1) + 2 \pmod{5} 27+65+2(mod5) \equiv 2 - 7 + 6 - 5 + 2 \pmod{5} 3(mod5). \equiv 3 \pmod{5}.

Thus, D is the correct answer.

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