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?
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