The method 100 students use to get to school and their grade level is shown below.

\begin{tabular}{|c|c|c|c|c|}
\hline & Drive & Bus & Walk & Total \\
\hline Sophomore & 2 & 25 & 3 & 30 \\
\hline Junior & 13 & 20 & 2 & 35 \\
\hline Senior & 25 & 5 & 5 & 35 \\
\hline Total & 40 & 50 & 10 & 100 \\
\hline
\end{tabular}

Find the probability that a student drives, given that they are a senior.

[tex]\[
P(\text{drives} \mid \text{senior}) = \frac{P(\text{drives and senior})}{P(\text{senior})} = \frac{25}{35}
\][/tex]



Answer :

Let's solve the problem step-by-step.

We need to find the probability that a student drives given that they are a senior. This probability is denoted as [tex]\( P(\text{drives} \mid \text{senior}) \)[/tex], which can be mathematically represented using the conditional probability formula:

[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{P(\text{drives and senior})}{P(\text{senior})} \][/tex]

First, we need to identify the required values from the given table.

1. Total number of seniors: This is the total number of students who are seniors.
- From the table, the total number of seniors is [tex]\( 35 \)[/tex].

2. Number of seniors who drive: This is the number of students who are seniors and also drive.
- From the table, the number of seniors who drive is [tex]\( 25 \)[/tex].

3. Probability of being a senior ( [tex]\( P(\text{senior}) \)[/tex] ): This is the proportion of students who are seniors out of the total number of students.
- The total number of students is [tex]\( 100 \)[/tex].
- Therefore, [tex]\( P(\text{senior}) = \frac{\text{Total number of seniors}}{\text{Total students}} = \frac{35}{100} \)[/tex].

4. Probability of driving and being a senior ( [tex]\( P(\text{drives and senior}) \)[/tex] ): This is the proportion of students who are both seniors and drive out of the total number of students.
- Therefore, [tex]\( P(\text{drives and senior}) = \frac{\text{Number of seniors who drive}}{\text{Total students}} = \frac{25}{100} \)[/tex].

To find [tex]\( P(\text{drives} \mid \text{senior}) \)[/tex]:

[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{P(\text{drives and senior})}{P(\text{senior})} = \frac{\frac{25}{100}}{\frac{35}{100}} = \frac{25}{35} \][/tex]

Simplifying [tex]\( \frac{25}{35} \)[/tex] gives us the final probability:

[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{25}{35} = \frac{5}{7} \approx 0.714 \][/tex]

Hence, the probability that a student drives given that they are a senior is approximately:

[tex]\[ P(\text{drives} \mid \text{senior}) = 0.714 \][/tex]

Or as a fraction:

[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{5}{7} \][/tex]