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2010 AMC 12B Problem 16

Problem 16 of 25IntermediateNumber TheoryCounting & Probability

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, \ldots, 2010\}. What is the probability that abc+ab+aabc+ab+a is divisible by 3?3?

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Solution

Factor abc+ab+a=a(bc+b+1).abc+ab+a=a(bc+b+1). Since 20102010 is a multiple of 3,3, each of a,b,ca, b, c is uniform modulo 3.3. If aa is divisible by 33 (probability 13\tfrac13), the product is divisible by 3.3. If aa is not divisible by 33 (probability 23\tfrac23), we need bc+b+1bc+b+1 to be divisible by 3.3. Checking residues, this holds exactly when (b,c)(1,1)(b,c)\equiv(1,1) or (2,0)(mod3),(2,0)\pmod3, a probability of 1313+1313=29.\tfrac13\cdot\tfrac13+\tfrac13\cdot\tfrac13=\tfrac29. The total probability is 13+2329=13+427=1327. \frac13+\frac23\cdot\frac29=\frac13+\frac{4}{27}=\frac{13}{27}. Thus, the correct answer is E.

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Concepts: modular arithmetic · basic probability · casework

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