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### Conditional Probability

Nadia's bookshelf contains 10 fiction books, 2 reference books, and 5 nonfiction books.

What is the probability that she randomly picks up a reference book and then, without replacing it, picks up a nonfiction book?

A. [tex]\frac{1}{289}[/tex]
B. [tex]\frac{10}{289}[/tex]
C. [tex]\frac{5}{136}[/tex]
D. [tex]\frac{1}{10}[/tex]


Sagot :

To determine the probability that Nadia randomly picks up a reference book and then, without replacing it, picks up a nonfiction book, we will calculate it step by step.

1. Count the total number of books:

- Fiction books: [tex]\(10\)[/tex]
- Reference books: [tex]\(2\)[/tex]
- Nonfiction books: [tex]\(5\)[/tex]

So, the total number of books is:
[tex]\[ 10 + 2 + 5 = 17 \][/tex]

2. Calculate the probability of picking a reference book first:

The probability [tex]\(P(A)\)[/tex] of picking a reference book out of the 17 books is:
[tex]\[ P(A) = \frac{\text{Number of reference books}}{\text{Total number of books}} = \frac{2}{17} = 0.11764705882352941 \][/tex]

3. Calculate the probability of picking a nonfiction book second, given that a reference book was picked first:

After picking a reference book, there are [tex]\(16\)[/tex] books left:
[tex]\[ \text{Remaining total books} = 17 - 1 = 16 \][/tex]

Now there are still [tex]\(5\)[/tex] nonfiction books because we did not pick any nonfiction book initially.

The probability [tex]\(P(B|A)\)[/tex] of picking a nonfiction book from the remaining [tex]\(16\)[/tex] books is:
[tex]\[ P(B|A) = \frac{\text{Number of nonfiction books}}{\text{Remaining total books}} = \frac{5}{16} = 0.3125 \][/tex]

4. Compute the combined probability of both events occurring:

According to the rule of conditional probability, the combined probability [tex]\(P(A \text{ and } B)\)[/tex] of both events (picking a reference book first and then picking a nonfiction book without replacement) is:
[tex]\[ P(A \text{ and } B) = P(A) \times P(B|A) = \left(\frac{2}{17}\right) \times \left(\frac{5}{16}\right) = 0.03676470588235294 \][/tex]

5. Convert the combined probability into fractional form to find the answer choice:

The calculated combined probability is [tex]\(0.03676470588235294\)[/tex], which in fractional form matches with:
[tex]\[ \frac{5}{136} \][/tex]

Hence, the probability that Nadia randomly picks up a reference book and then, without replacing it, picks up a nonfiction book is [tex]\(\boxed{\frac{5}{136}}\)[/tex].
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