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In how many ways can 4 balls be placed in 4 boxes if 3 balls are indistinguishably white, 1 ball is black, and the boxes are indistinguishable?

 Oct 19, 2024
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To solve the problem, we need to find how many distinct ways we can distribute 4 balls (3 indistinguishable white balls and 1 distinguishable black ball) into 4 indistinguishable boxes.

 

Let's denote the number of white balls in each box by a1, a2, a3, and a4, where ai represents the number of white balls in box i and we need to find non-negative integer solutions to the equation:

a1+a2+a3+a4=3

We also have to take into account the placement of the black ball after distributing the white balls.

### Step 1: Count the distributions of white balls

First, we will enumerate the different partitions of the number 3, corresponding to how we can allocate 3 indistinguishable white balls into indistinguishable boxes. The partitions of 3 are:

1. 3 (All three white balls in one box)


2. 2+1 (Two white balls in one box and one white ball in another)


3. 1+1+1 (One white ball in each of three boxes)

Now we can list these partitions in a way that clearly defines the placement in indistinguishable boxes:

- **Partition 3**: (3)


- **Partition 2+1**: (2, 1)


- **Partition 1+1+1**: (1, 1, 1)

### Step 2: Place the black ball

Now we need to consider how the black ball can be placed in relation to each distribution of the white balls:

1. **For the distribution of 3**:


- There is only 1 box with all 3 white balls. When we add the black ball, it can go into this box or one of the remaining (empty) boxes.


- Total configurations = 1 (black in the box) + 3 (black in empty boxes) = **1 way**.

2. **For the distribution of 2+1**:


- We have one box with 2 white balls and one box with 1 white ball.


- Possible locations for the black ball:


- It can go into the box with 2 white balls (1 way)


- It can go into the box with 1 white ball (1 way)


- It can go into one of the empty boxes (2 empty boxes)


- Therefore, total configurations = 1 + 1 + 2 = **4 ways**.

3. **For the distribution of 1+1+1**:


- There are 3 boxes, each containing 1 white ball.


- The black ball can go into any of these 3 boxes (there are no empty boxes since each has a white ball).


- Total configurations = 3 (the black ball can go into any of the three boxes) = **3 ways**.

### Step 3: Sum the possibilities

Finally, we sum all the possible placements derived from each distribution:

1+4+3=8

Thus, the total number of ways to place 4 balls (3 indistinguishably white and 1 distinguishable black) into 4 indistinguishable boxes is **8**.

Therefore, the answer is:

8

 Oct 19, 2024

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