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

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

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

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

Round to the nearest hundredth.


Sagot :

To solve this problem, we need to find the probability that a student drives given that they are a senior. This is often written as [tex]\( P(\text{drives} \mid \text{senior}) \)[/tex].

We are given a table that shows the distribution of how the 100 students get to school according to their grade levels. Here is the table again for convenience:

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

### Step-by-Step Solution:

1. Determine [tex]\( P(\text{senior}) \)[/tex]:

We need the probability that a student is a senior. We have 35 seniors out of a total of 100 students.
[tex]\[ P(\text{senior}) = \frac{\text{Number of seniors}}{\text{Total number of students}} = \frac{35}{100} = 0.35 \][/tex]

2. Determine [tex]\( P(\text{drives and senior}) \)[/tex]:

Next, we need the probability that a student both drives and is a senior. There are 25 such students out of a total of 100.
[tex]\[ P(\text{drives and senior}) = \frac{\text{Number of seniors who drive}}{\text{Total number of students}} = \frac{25}{100} = 0.25 \][/tex]

3. Calculate [tex]\( P(\text{drives} \mid \text{senior}) \)[/tex]:

We use the conditional probability formula:
[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{P(\text{drives and senior})}{P(\text{senior})} \][/tex]

Substituting in the values we have determined:
[tex]\[ P(\text{drives} \mid \text{senior}) = \frac{0.25}{0.35} \approx 0.7142857142857143 \][/tex]

4. Round to the nearest hundredth:

The last step is to round the result to the nearest hundredth:
[tex]\[ 0.7142857142857143 \approx 0.71 \][/tex]

So, the probability that a student drives given that they are a senior is [tex]\( \boxed{0.71} \)[/tex].
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