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A 3x3x3 wooden cube is painted of all six faces, then cut into 27 unit cubes. One unit cube is randomly selected and rolled. After it is rolled, 5 of the 6 faces of the cube are visible, with the face on the bottom unseen. What is the probability that exactly one of the five faces is painted. 

Guest Jun 26, 2018
 #1
avatar+867 
+3

We first need to determine how the 27 unit cubes are painted. 

 

Number of Painted Sides Number of Cubes
3 The 8 corner cubes

2

12 non-corner edge cubes
1 6 visible center cubes
0 1 non-visible center cube

 

Cases:

 

Case 1: We select a cube with 0 painted sides

 

It is not possible to see a painted side, since none are painted. 

 

Case 2: We select a cube with 1 painted sides

 

The roll is successful as long as an unpainted side is on the bottom. 

 

Case 3: We select a cube with 2 painted sides

 

The roll is successful as long as a painted side is on the bottom. 

 

Case 4: We select a cube with 3 painted sides

 

It is not possible to see only one painted side, since only on can be hidden, but the other two cannot. Therefore, we will see more than one painted side. 

 

Probabilty:

 

We only need to consider cases 1 and 2. 

 

Case 2:

 

6 sides to a cube, 5 unpainted and 1 painted. 

 

We need an unpainted side on the bottom, 5/6. 

 

Case 3: 

 

We need a painted side of the bottom, 2/6. 

 

\(\frac{12}{27}\cdot\frac26+\frac{6}{27}\cdot\frac56=\boxed{\frac13}\)

 

I hope this helped,

 

Gavin

GYanggg  Jun 26, 2018
 #2
avatar+87334 
+2

Thanks, Gavin....!!!!

 

Here's another way to see this....

 

Note that there are 9  faces on each side of the large cube that are painted....so....the total number of painted faces is  just  9 * 6  =  54  painted faces

 

And the total number  of faces on the smaller cubes  is just  27 cubes * 6  faces on each  = 162

 

If we put all the cubes in a bag and selected only one, the probability that it would have a painted face  is just  54  /162  =  1 / 3

 

 

cool cool cool

CPhill  Jun 26, 2018
edited by CPhill  Jun 26, 2018
 #3
avatar+867 
+1

Hey CPhill!

 

I don't think your answer accounts for this part of the question: 

 

After it is rolled, 5 of the 6 faces of the cube are visible, with the face on the bottom unseen. What is the probability that exactly one of the five faces is painted.

 

In your case there isn't the rolling process, hence the "5 of the 6 faces of the cube are visible, with the face on the bottom unseen" part was neglected. 

 

I'm still trying to find out why you also got the same answer. 

 

I might have missed something about your solution and I'm the one at fault, and you solution is actually correct. 

GYanggg  Jun 26, 2018
 #4
avatar+87334 
0

Yeah...I see what you mean, Gavin...your answer is probably better....mine might just be coincindence....I don't know....!!!

 

 

cool cool cool

CPhill  Jun 26, 2018
edited by CPhill  Jun 26, 2018

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