Question

Stan guessed on all 10 questions of a multiple-choice quiz. Each question has 4 answer choices. What is the probability that he got at least 2 questions correct? Round the answer to the nearest thousandth.
0.211
0.244
0.756
0.944

Answer 1

Answer:

C

Step-by-step explanation:

Answer 2

ANSWER
0.756

EXPLANATION

Let x represent number of correct answers.

We can find

   $P(x = 0 \text{ or } x = 1)$

which leads us to

   $P(x\ge 2) = 1 - P(x = 0\text{ or }x = 1)$

which uses the compliment of $P(x = 0\text{ or }x = 1)$ to find the probability of getting at least 2 questions correct.

Note that since $P(x=0)$ and $P(x=1)$ are mutually exclusive, we have

   $P(x = 0\text{ or }x = 1) = P(x=0) + P(x=1)$

Then $P(x=0) = 3^{10}/4^{10} = (3/4)^{10}$ as we have $3^{10}$ to answer the questions incorrectly divided by the total $4^{10}$ to answer the questions.

Exactly 1 answer correct: $P(x=1) = {}_{10}C_1 \cdot(3/4)^9(1/4)^1$

Therefore

   $\begin{aligned} P(x \ge 2) &= 1 - P(x = 0\text{ or }x =1) \\ &=1 - \big[P(x=0) + P(x=1)\big] \\ &=1 - \left[ (3/4)^{10} + {}_{10}C_1 \cdot (3/4)^9(1/4)^1 \right] \\ &\approx 0.756 \end{aligned}$