2023 AMC 10A Problems/Problem 18

Problem

A rhombic dodecahedron is a solid with $12$ congruent rhombus faces. At every vertex, $3$ or $4$ edges meet, depending on the vertex. How many vertices have exactly $3$ edges meet?

$\textbf{(A) }5\qquad\textbf{(B) }6\qquad\textbf{(C) }7\qquad\textbf{(D) }8\qquad\textbf{(E) }9$

Solution 1

Note Euler's formula where $V+F-E=2$ . There are $12$ faces and the number of edges is $24$ because there are 12 faces each with four edges and each edge is shared by two faces. Now we know that there are $14$ vertices on the figure. Let $A$ be the number of vertices with degree 3 and $B$ be the number of vertices with degree 4. $A+B=14$ is our first equation. Now note that the sum of the degrees of all the points is twice the number of edges. Now we know $3A+4B=48$ . Solving this system of equations gives $B = 6$ and $A = 8$ so the answer is $\boxed{\textbf{(D) }8}$ . ~aiden22gao ~zgahzlkw (LaTeX)

Solution 2: Quick solution

Let $x$ be the number of vertices with three edges, and $y$ be the number of vertices with four edges. Since there are $\frac{4*12}{2}=24$ edges on the polyhedron, we can see that $\frac{3x+4y}{2}=24$ . Then, $3x+4y=48$ . Notice that by testing the answer choices, (D) is the only one that yields an integer solution for $y$ . Thus, the answer is $\boxed{\textbf{(D) }8}$ .

Solution 2

With $12$ rhombi, there are $4\cdot12=48$ total boundaries. Each edge is used as a boundary twice, once for each face on either side. Thus we have $\dfrac{48}2=24$ total edges.

Let $A$ be the number of vertices with $3$ edges (this is what the problem asks for) and $B$ be the number of vertices with $4$ edges. We have $3A + 4B = 48$ .

Euler's formula states that, for all convex polyhedra, $v-e+f=2$ . In our case, $v-24+12=2\implies v=14.$ We know that $A+B$ is the total number of vertices as we are given that all vertices are connected to either $3$ or $4$ edges. Therefore, $A+B=14.$

We now have a system of two equations. There are many ways to solve for $A$ ; choosing one yields $A=\boxed{\textbf{(D) }8}$ .

Even without Euler's formula, we can do a bit of answer guessing. From $3A+4B=48$ , we take mod $4$ on both sides.

$3A+4B\equiv48~(\mod4)$ $3A\equiv0~(\mod4)$

We know that $3A$ must be divisible by $4$ . We know that the factor of $3$ will not affect the divisibility by $4$ of $3A$ , so we remove the $3$ . We know that $A$ is divisible by $4$ . Checking answer choices, the only one divisible by $4$ is indeed $A=\boxed{\textbf{(D) }8}$ .

~Technodoggo ~zgahzlkw (small edits)

Solution 3

Note that Euler's formula is $V+F-E=2$ . We know $F=12$ from the question. We also know $E = \frac{12 \cdot 4}{2} = 24$ because every face has $4$ edges and every edge is shared by $2$ faces. We can solve for the vertices based on this information.

Using the formula we can find: $V + 12 - 24 = 2$ $V = 14$ Let $t$ be the number of vertices with $3$ edges and $f$ be the number of vertices with $4$ edges. We know $t+f = 14$ from the question and $3t + 4f = 48$ . The second equation is because the total number of points is $48$ because there are 12 rhombuses of $4$ vertices. Now, we just have to solve a system of equations. $3t + 4f = 48$ $3t + 3f = 42$ $f = 6$ $t = 8$ Our answer is simply just $t$ , which is $\boxed{\textbf{(D) }8}$ ~musicalpenguin

Solution 4

Each of the twelve rhombuses has two pairs of angles across from each other that must be the same. If both pairs of angles occur at $4$ -point intersections, we have a grid of squares. If both occur at $3$ -point intersections, we would have a cube with six square faces. Therefore, two of the points must occur at a $3$ -point intersection and two at a $4$ -point intersection.

Since each $3$ -point intersection has $3$ adjacent rhombuses, we know the number of $3$ -point intersections must equal the number of $3$ -point intersections per rhombus times the number of rhombuses over $3$ . Since there are $12$ rhombuses and two $3$ -point intersections per rhombus, this works out to be:

$\frac{2*12}{3}$

Hence: $\boxed{\textbf{(D) }8}$ ~hollph27

Solution 5 (Based on previous knowledge)

Note that a rhombic dodecahedron is formed when a cube is turned inside out (as seen here), thus there are 6 4-vertices (corresponding to each face of the cube) and 8 3-vertices (corresponding to each corner of the cube). Thus the answer is $\boxed{\textbf{(D) }8}$

Solution 6 (Using Answer Choices)

Let $m$ be the number of $4$ -edge vertices, and $n$ be the number of $3$ -edge vertices. The total number of vertices is $m+n$ . Now, we know that there are $4 \cdot 12 = 48$ vertices, but we have overcounted. We have overcounted $m$ vertices $3$ times and overcounted $n$ vertices $2$ times. Therefore, we subtract $3m$ and $2n$ from $48$ and set it equal to our original number of vertices. $48 - 3m - 2n = m+n$ $4m + 3n = 48$ From here, we reduce both sides modulo $4$ . The $4m$ disappears, and the left hand side becomes $3n$ . The right hand side is $0$ , meaning that $3n$ must be divisible by $4$ . Looking at the answer choices, this is only possible for $n = \boxed{8}$ .

-DEVSAXENA

(Isn't this the same as the last half of Solution 2?)

2023 AMC 10A (Problems • Answer Key • Resources)
Preceded by Problem 17	Followed by Problem 19
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