2010 AMC 12B Problems/Problem 16

Problem 16

Positive integers $a$ , $b$ , and $c$ are randomly and independently selected with replacement from the set $\{1, 2, 3,\dots, 2010\}$ . What is the probability that $abc + ab + a$ is divisible by $3$ ?

$\textbf{(A)}\ \dfrac{1}{3} \qquad \textbf{(B)}\ \dfrac{29}{81} \qquad \textbf{(C)}\ \dfrac{31}{81} \qquad \textbf{(D)}\ \dfrac{11}{27} \qquad \textbf{(E)}\ \dfrac{13}{27}$

Solution

We group this into groups of $3$ , because $3|2010$ .

If $3|a$ , we are done. There is a probability of $\frac{1}{3}$ that that happens.

Otherwise, we have $3|bc+b+1$ , which means that $b(c+1) \equiv 2\pmod{3}$ . So either $b \equiv 1 \pmod{3}, c \equiv 1 \pmod{3}$ or $b \equiv 2 \pmod {3}, c \equiv 0 \pmod 3$ which will lead to the property being true. There are a $\frac{1}{3}\cdot\frac{1}{3}=\frac{1}{9}$ chance for each bundle of cases to be true. Thus, the total for the cases is $\frac{2}{9}$ . But we have to multiply by $\frac{2}{3}$ because this only happens with a $\frac{2}{3}$ chance. So the total is actually $\frac{4}{27}$ .

The grand total is $\frac{1}{3} + \frac{4}{27} = \boxed{\text{(E) }\frac{13}{27}.}$

2010 AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 15	Followed by Problem 17
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2010 AMC 12B Problems/Problem 16

Problem 16

Solution

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