Johnni guessed on all 8 questions of a multiple-choice quiz. Each question has 4 answer choices. What is the probability that she got exactly 3 questions correct? Round the answer to the nearest thousandth.

[tex]\[
\begin{array}{c}
P(k \text{ successes }) = { }_n C _k p^k(1-p)^{n-k} \\
{ }_n C _k = \frac{n!}{(n-k)!\cdot k!}
\end{array}
\][/tex]

A. 0.004
B. 0.208
C. 0.422
D. 0.792



Answer :

First, let's understand the context and variables involved in this problem.

Johnni is answering 8 multiple-choice questions by guessing. Each question has 4 answer choices, so the probability of guessing any question correctly (p) is:

[tex]\[ p = \frac{1}{4} = 0.25 \][/tex]

We are interested in finding the probability that Johnni gets exactly 3 questions correct out of 8. This situation can be modeled using the binomial probability formula:

[tex]\[ P(k \text{ successes}) = {}_n C_k \cdot p^k \cdot (1-p)^{n-k} \][/tex]

where:
- [tex]\( n \)[/tex] is the total number of questions (8 in this case).
- [tex]\( k \)[/tex] is the number of correct answers we are finding the probability for (3 in this case).
- [tex]\( p \)[/tex] is the probability of guessing a question correctly (0.25).
- [tex]\( 1-p \)[/tex] is the probability of guessing a question incorrectly (0.75).
- [tex]\( {}_n C_k \)[/tex] is the binomial coefficient, calculated as:

[tex]\[ {}_n C_k = \frac{n!}{k! \cdot (n-k)!} \][/tex]

For our problem:
[tex]\[ n = 8 \][/tex]
[tex]\[ k = 3 \][/tex]

Now, we'll calculate [tex]\( {}_8 C_3 \)[/tex]:

[tex]\[ {}_8 C_3 = \frac{8!}{3! \cdot (8-3)!} = \frac{8!}{3! \cdot 5!} \][/tex]

[tex]\[ 8! = 8 \times 7 \times 6 \times 5 \times 4 \times 3 \times 2 \times 1 \][/tex]
[tex]\[ 3! = 3 \times 2 \times 1 \][/tex]
[tex]\[ 5! = 5 \times 4 \times 3 \times 2 \times 1 \][/tex]

So:
[tex]\[ {}_8 C_3 = \frac{8 \times 7 \times 6 \times 5!}{3 \times 2 \times 1 \times 5!} = \frac{8 \times 7 \times 6}{3 \times 2 \times 1} = \frac{336}{6} = 56 \][/tex]

Now we can use the binomial formula:

[tex]\[ P(3 \, \text{successes}) = 56 \cdot (0.25)^3 \cdot (0.75)^{8-3} \][/tex]

Calculate [tex]\( (0.25)^3 \)[/tex] and [tex]\( (0.75)^5 \)[/tex]:

[tex]\[ (0.25)^3 = 0.25 \times 0.25 \times 0.25 = 0.015625 \][/tex]
[tex]\[ (0.75)^5 = 0.75 \times 0.75 \times 0.75 \times 0.75 \times 0.75 = 0.2373046875 \][/tex]

Now, multiply these values with the binomial coefficient:

[tex]\[ P(3 \, \text{successes}) = 56 \cdot 0.015625 \cdot 0.2373046875 \][/tex]

[tex]\[ P(3 \, \text{successes}) = 56 \cdot 0.0037060546875 = 0.2076416015625 \][/tex]

Finally, round this result to the nearest thousandth:

[tex]\[ P(3 \, \text{successes}) \approx 0.208 \][/tex]

Therefore, the probability that Johnni got exactly 3 questions correct is approximately [tex]\( 0.208 \)[/tex]. Thus, the correct answer to the question is:

[tex]\[ \boxed{0.208} \][/tex]

Answer:

Step-by-step explanation:

To find the probability that Johnni got exactly 3 out of 8 questions correct on a multiple-choice quiz, we use the binomial probability formula:

(

 successes

)

=

(

)

(

1

)

P(k successes)=(

k

n

)p

k

(1−p)

n−k

where:

=

8

n=8 (total number of questions),

=

3

k=3 (number of correct answers),

=

1

4

p=

4

1

 (probability of getting a question correct),

1

=

3

4

1−p=

4

3

 (probability of getting a question incorrect).

First, calculate the binomial coefficient

(

8

3

)

(

3

8

):

(

8

3

)

=

8

!

3

!

(

8

3

)

!

=

8

!

3

!

5

!

=

8

×

7

×

6

3

×

2

×

1

=

56

(

3

8

)=

3!(8−3)!

8!

=

3!⋅5!

8!

=

3×2×1

8×7×6

=56

Next, calculate

p

k

 and

(

1

)

(1−p)

n−k

:

=

(

1

4

)

3

=

1

64

p

k

=(

4

1

)

3

=

64

1

(

1

)

=

(

3

4

)

5

=

243

1024

(1−p)

n−k

=(

4

3

)

5

=

1024

243

Combine these to find the probability:

(

3

 correct

)

=

(

8

3

)

×

×

(

1

)

=

56

×

1

64

×

243

1024

P(3 correct)=(

3

8

)×p

k

×(1−p)

n−k

=56×

64

1

×

1024

243

Calculate the product:

56

×

1

64

=

56

64

=

7

8

56×

64

1

=

64

56

=

8

7

7

8

×

243

1024

=

7

×

243

8

×

1024

=

1701

8192

8

7

×

1024

243

=

8×1024

7×243

=

8192

1701

Convert this to decimal form:

1701

8192

0.208

8192

1701

≈0.208

Thus, the probability that Johnni got exactly 3 questions correct is approximately

0.208

0.208