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A box contains four red balls and eight black balls. Two balls are randomly chosen from the box and are not replaced. Let event [tex]\( B \)[/tex] be choosing a black ball first and event [tex]\( R \)[/tex] be choosing a red ball second.

What are the following probabilities?
[tex]\[
\begin{array}{l}
P(B)=\square \\
P(R \mid B)=\square \\
P(B \cap R)=\square
\end{array}
\][/tex]

The probability that the first ball chosen is black and the second ball chosen is red is about [tex]\(\square\)[/tex] percent.


Sagot :

Let's solve this problem step by step.

We have a box containing four red balls and eight black balls, making a total of twelve balls.

### Finding [tex]\( P(B) \)[/tex]

1. Probability of choosing a black ball first ([tex]\( P(B) \)[/tex]):

Total number of balls = 12

Number of black balls = 8

The probability of choosing a black ball first is given by the ratio of the number of black balls to the total number of balls.

[tex]\[ P(B) = \frac{\text{Number of black balls}}{\text{Total number of balls}} = \frac{8}{12} = \frac{2}{3} \approx 0.6667 \][/tex]

### Finding [tex]\( P(R \mid B) \)[/tex]

2. Probability of choosing a red ball second given that the first ball was black ([tex]\( P(R \mid B) \)[/tex]):

After choosing a black ball, one black ball is removed from the total, leaving us with 11 balls in total (4 red balls and 7 black balls remaining).

The probability of choosing a red ball out of these remaining balls is given by the ratio of the number of red balls to the new total number of balls.

[tex]\[ P(R \mid B) = \frac{\text{Number of red balls}}{\text{Remaining number of balls}} = \frac{4}{11} \approx 0.3636 \][/tex]

### Finding [tex]\( P(B \cap R) \)[/tex]

3. Probability of both events happening ([tex]\( P(B \cap R) \)[/tex]):

The probability of both events happening is the product of the probability of each individual event happening sequentially. This is the joint probability of choosing a black ball first and a red ball second.

[tex]\[ P(B \cap R) = P(B) \times P(R \mid B) \][/tex]

Substituting in the values we found earlier:

[tex]\[ P(B \cap R) = \left(\frac{2}{3}\right) \times \left(\frac{4}{11}\right) \approx 0.6667 \times 0.3636 \approx 0.2424 \][/tex]

### Converting to Percentage

4. Convert the probability to a percentage:

[tex]\[ P(B \cap R) \approx 0.2424 \][/tex]

To convert this to a percentage, multiply by 100:

[tex]\[ P(B \cap R) \times 100 \approx 24.24\% \][/tex]

### Summary

- The probability of choosing a black ball first ([tex]\( P(B) \)[/tex]) is approximately 0.6667.
- The probability of choosing a red ball second given the first was black ([tex]\( P(R \mid B) \)[/tex]) is approximately 0.3636.
- The probability of both events happening ([tex]\( P(B \cap R) \)[/tex]) is approximately 0.2424.
- The probability that the first ball chosen is black and the second ball chosen is red is about 24.24 percent.