2021 AMC 10A Fall Problem 12

Below is the professionally curated solution for Problem 12 of the 2021 AMC 10A Fall, 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.

All of the real AMC 8, AMC 10, AMC 12, and AIME problems in our complete solution collection are used with official legal permission of the Mathematical Association of America (MAA).

Concepts:number basemodular arithmetic

Difficulty rating: 1070

12.

The base-nine representation of the number NN is 27,006,000,052nine.27,006,000,052_{\text{nine}}. What is the remainder when NN is divided by 5?5?

00

11

22

33

44

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.

Problem 12 in Other Years