2014 AMC 10A Problems/Problem 17

Problem

Three fair six-sided dice are rolled. What is the probability that the values shown on two of the dice sum to the value shown on the remaining die?

$\textbf{(A)}\ \dfrac16\qquad\textbf{(B)}\ \dfrac{13}{72}\qquad\textbf{(C)}\ \dfrac7{36}\qquad\textbf{(D)}\ \dfrac5{24}\qquad\textbf{(E)}\ \dfrac29$

Video Solution

https://youtu.be/5UojVH4Cqqs?t=702

~ pi_is_3.14

Solution 1

First, we note that there are $1, 2, 3, 4,$ and $5$ ways to get sums of $2, 3, 4, 5, 6$ respectively--this is not too hard to see. With any specific sum, there is exactly one way to attain it on the other die. This means that the probability that two specific dice have the same sum as the other is $\dfrac16 \left( \dfrac{1+2+3+4+5}{36}\right) = \dfrac{5}{72}.$ Since there are $\dbinom31$ ways to choose which die will be the one with the sum of the other two, our answer is $3 \cdot \dfrac{5}{72} = \boxed{\textbf{(D)} \: \dfrac{5}{24}}$ .

Solution 2 (Bashy)

$(1, 2, 3); (1, 3, 4); (1, 4, 5); (1, 5, 6); (2, 3, 5); (2, 4, 6)$ have $6$ ways to rearrange them for a total of $36$ ways. $(1, 1, 2); (2, 2, 4); (3, 3, 6)$ have $3$ ways to rearrange them for a total of $9$ ways. Adding them up, we get $45$ ways. We have to divide this values by $6^3$ because there are 3 dice. $\dfrac{45}{216}=\boxed{\dfrac{5}{24}}$ .

~MathFun1000

Solution 3 (Summary of Solution 2)

Start by listing all possible sums. Then find the ways to arrange them and sum them up and divide by $6^3$ for 3 dice. $\dfrac{45}{216}$ is $\boxed{\textbf{(D)} \: \dfrac{5}{24}}$ .

-aopspandy

Solution 4 (enumeration and casework)

There are $3$ ways to pick the dice which is the sum. From here proceed with "casework":

If the sum is 1, there are $0$ ways because the minimum sum is $2$ .
If the sum is 2, there is $1$ way (namely $(1, 1)$ ).
If the sum is 3, there are $2$ ways (namely $(1, 2)$ and $(2, 1)$ ).
If the sum is 4, there are $3$ ways (namely $(1, 3)$ , $(3, 1)$ , and $(2, 2)$ ).

We start noticing that for a sum of $n$ there are $n-1$ cases, but let's keep going to verify.

If the sum is 5, there are $4$ ways (namely $(1, 4)$ , $(4, 1)$ , $(2, 3)$ , and $(3, 2)$ ).
If the sum is 6, there are $5$ ways (namely $(1, 5)$ , $(5, 1)$ , $(2, 4)$ , $(4, 2)$ , and $(3, 3)$ ).

We have exhausted all cases because there is no way a dice can roll a $7$ . Summing, there are 15 ways here. To summarize: there are $3$ ways to pick the dice that is the sum, and $15$ ways to achieve the sum, for a total of $45$ configurations. There are $6\cdot6\cdot6=216$ total configurations, and the probability is $\frac{45}{216}=\frac{5}{24} \Longrightarrow \boxed{\textbf{(D)} \frac{5}{24}}$ .

~JH. L

2014 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 16	Followed by Problem 18
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