Count Painted Blocks in a Large Cube

MathematicsGeometry and VolumeMediumSTEM

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Stacie builds a cube using 343 blocks of wood. She decides to paint the cube green. How many of the wooden blocks will have at least one side painted green?

A) 218

B) 125

C) 245

D) 238

E) 105

Animated Video Solution

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Step by Step Written Solution

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Step 1

Hi Ersan, let's solve this problem about a large cube made from smaller wooden blocks.

Problem Analysis

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Step 2

Stacie builds a cube using three hundred forty-three small blocks. We need to find how many of these blocks have at least one side painted green after the large cube is painted.

Total blocks: $V = 343$

Find: Blocks with $\ge 1$ side painted.

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Step 3

First, we need to find the dimensions of the large cube. Since it is a cube made of n cubed blocks, we take the cube root of three hundred forty-three.

Step 1: Find dimensions of the cube

$$n = \sqrt[3]{343}$$
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Step 4

Because seven times seven is forty-nine, and forty-nine times seven is three hundred forty-three, we know the side length is seven.

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Step 5

This means the large cube consists of seven blocks along each edge.

n = 7
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Step 6

A common way to find the number of painted blocks is to subtract the unpainted blocks from the total number of blocks.

Step 2: Strategy

$$\text{Painted Blocks} = \text{Total Blocks} - \text{Unpainted Blocks}$$
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Step 7

The unpainted blocks are the ones completely inside the cube, forming a smaller inner cube.

Inner unpainted cube dimensions: $(n-2) \times (n-2) \times (n-2)$

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About This Question

Subject
Mathematics
Topic
Geometry and Volume
Difficulty
Medium
Exam
STEM
Question Type
Multiple Choice

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