2010 AMC 10B Problem 18

Attempt Problem 18 of the 2010 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 2010 AMC 10B solutions, or check the answer key.

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18.

Positive integers a,a, b,b, and cc are randomly and independently selected with replacement from the set {1,2,3,,2010}.\{1, 2, 3,\dots, 2010\}.

What is the probability that abc+ab+aabc + ab + a is divisible by 3?3?

13\dfrac{1}{3}

2981\dfrac{29}{81}

3181\dfrac{31}{81}

1127\dfrac{11}{27}

1327\dfrac{13}{27}

Answer: E
Concepts:modular arithmeticbasic probabilitycasework
Difficulty rating: 1660
Solution:

Note that abc+ab+a=a(bc+b+1). abc + ab + a = a(bc + b + 1). This means that if aa is divisible by 3,3, the whole expression is as well.

Since 20102010 is divisible by 3,3, we have that aa is divisible by 33 with probability 13.\frac{1}{3}.

Now consider aa not divisible by 3.3. For the expression to be divisible by 3,3, we must have that bc+b+1bc + b + 1 is divisible by 3.3.

This means that bc+b=b(c+1)2(mod3). bc + b = b(c + 1) \equiv 2 \pmod{3}.

The only possibility for this is that one of the factors is 22 mod 33 and the other is 11 mod 3.3.

For each of the two cases, there is a 13\frac{1}{3} chance that each of the factors is the desired modulus, for a probability of 1313=19.\dfrac{1}{3} \cdot \dfrac{1}{3} = \dfrac{1}{9}.

There are two cases, which means that this happens with a 29\dfrac{2}{9} probability.

The total probability is then 131+2329=1327. \dfrac{1}{3} \cdot 1 + \dfrac{2}{3} \cdot \dfrac{2}{9} = \dfrac{13}{27}.

Thus, E is the correct answer.

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