2011 AMC 10B Problem 23

Attempt Problem 23 of the 2011 AMC 10B 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 2011 AMC 10B solutions, or check the answer key.

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23.

What is the hundreds digit of 20112011?2011^{2011}?

1 1

4 4

5 5

6 6

9 9

Answer: D
Concepts:modular exponentiationbinomial theorem
Difficulty rating: 1820
Solution:

Because 201111(mod1000)2011\equiv11\pmod{1000}, it is enough to find 112011=(10+1)201111^{2011}=(10+1)^{2011} modulo 10001000.

All terms with 10310^3 or higher are divisible by 10001000, so only the first three terms matter: 1+201110+(20112)102.1+2011\cdot10+\binom{2011}{2}10^2.

Modulo 10001000, this is 1+1110+2011201021001+110+500=611 \begin{aligned} &1+11\cdot10+\dfrac{2011\cdot2010}{2}\cdot100 \\ &\quad \equiv1+110+500=611 \end{aligned} .

The hundreds digit is therefore 66.

Thus, D is the correct answer.

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