2021 AMC 10A Spring Problem 11

Below is the video solution and professionally curated solution for Problem 11 of the 2021 AMC 10A Spring, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Spring solutions, or check the answer key.

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Concepts:number basedivisibility

Difficulty rating: 1020

11.

For which of the following integers bb is the base-bb number 2021b221b2021_b - 221_b not divisible by 3?3?

33

44

66

77

88

Video solution:
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Written solution:

We can express this expression in base 1010 using the definition of bases: 2b3+2b+12b22b1 2b^3 + 2b + 1 - 2b^2 - 2b - 1 =2b32b2=2b2(b1).= 2b^3 - 2b^2 = 2b^2(b - 1).

For this to be divisible by 3,3, either bb or b1b - 1 must be divisible by 3.3.

The only answer choice that satisfies neither of these conditions is 8.8.

Thus, E is the correct answer.

Problem 11 in Other Years