2006 AMC 12B Problems/Problem 17

Problem

For a particular peculiar pair of dice, the probabilities of rolling $1$ , $2$ , $3$ , $4$ , $5$ and $6$ on each die are in the ratio $1:2:3:4:5:6$ . What is the probability of rolling a total of $7$ on the two dice?

$\mathrm{(A)}\ \frac 4{63} \qquad \mathrm{(B)}\ \frac 18 \qquad \mathrm{(C)}\ \frac 8{63} \qquad \mathrm{(D)}\ \frac 16 \qquad \mathrm{(E)}\ \frac 27$

Solution

The probability of getting an $x$ on one of these dice is $\frac{x}{21}$ .

The probability of getting $1$ on the first and $6$ on the second die is $\frac 1{21}\cdot\frac 6{21}$ . Similarly we can express the probabilities for the other five ways how we can get a total $7$ . (Note that we only need the first three, the other three are symmetric.)

Summing these, the probability of getting a total $7$ is: $2\cdot\left( \frac 1{21}\cdot\frac 6{21} + \frac 2{21}\cdot\frac 5{21} + \frac 3{21}\cdot\frac 4{21} \right) = \frac{56}{441} = \boxed{\frac{8}{63}}$

2006 AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
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2006 AMC 12B Problems/Problem 17

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