2018 AMC 10B Problems/Problem 9

Problem

The faces of each of $7$ standard dice are labeled with the integers from $1$ to $6$ . Let $p$ be the probabilities that when all $7$ dice are rolled, the sum of the numbers on the top faces is $10$ . What other sum occurs with the same probability as $p$ ?

$\textbf{(A)} \text{ 13} \qquad \textbf{(B)} \text{ 26} \qquad \textbf{(C)} \text{ 32} \qquad \textbf{(D)} \text{ 39} \qquad \textbf{(E)} \text{ 42}$

Solution 1

It can be seen that the probability of rolling the smallest number possible is the same as the probability of rolling the largest number possible, the probability of rolling the second smallest number possible is the same as the probability of rolling the second largest number possible, and so on. This is because the number of ways to add a certain number of ones to an assortment of 7 ones is the same as the number of ways to take away a certain number of ones from an assortment of 7 6s.

So, we can match up the values to find the sum with the same probability as 10. We can start by noticing that 7 is the smallest possible roll and 42 is the largest possible role. The pairs with the same probability are as follows:

(7, 42), (8, 41), (9, 40), (10, 39), (11, 38)...

However, we need to find the number that matches up with 10. So, we can stop at (10, 39) and deduce that the sum with equal probability as 10 is 39. So, the correct answer is $\boxed{\textbf{(D)} \text{ 39}}$ , and we are done.

Written By: Archimedes15

Solution 2

Let's call the unknown value $x$ . By symmetry, we realize that the difference between 10 and the minimum value of the rolls is equal to the difference between the maximum and $x$ . So,

$10 - 7 = 42- x$

$x = 39$ and our answer is $\boxed{\textbf{(D)} \text{ 39}}$ By: Soccer_JAMS

Solution 3 (Simple Logic)

For the sums to have equal probability, the average sum of both sets of 7 dies has to be (6+1) x 7 = 49. Since having 10 is similar to not having 10, you just subtract 10 from the expected total sum. 49 - 10 = 39 so the answer is $\boxed{\textbf{(D)} \text{ 39}}$

By: epicmonster

Solution 4

The expected value of the sums of the die rolls is $3.5*7=24.5$ , and since the probabilities should be distributed symmetrically on both sides of $24.5$ , the answer is $24.5+24.5-10=39$ , which is $\boxed{\textbf{(D)} \text{ 39}}$ .

By: dajeff

Note

Calculating the probability of getting a sum of $10$ is also easy. There are $3$ cases:

Case $1$ : $\{1,1,1,1,1,1,4\}$

$\frac{7!}{6!}=7$ cases

Case $2$ : $\{1,1,1,1,1,2,3\}$

$\frac{7!}{5!}=6*7=42$ cases

Case $3$ : $\{1,1,1,1,2,2,2\}$

$\frac{7!}{4!3!}=5*7=35$ cases

The probability is ${84 \over 6^7} = \frac{14}{6^6}$ .

Calculating $6^6$ :

$6^6=(6^3)^2=216^2=46656$

Therefore, the probability is ${14 \over 46656} = \boxed{{7 \over 23328}}$

~Zeric Hang

2018 AMC 10B (Problems • Answer Key • Resources)
Preceded by Problem 8	Followed by Problem 10
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