2010 AMC 10B Problem 18

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Concepts:modular arithmeticbasic probabilitycasework

Difficulty rating: 1660

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}

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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