To solve this problem, we can model it using a binomial probability distribution since each question has only two possible outcomes for the person guessing randomly: a correct or incorrect answer. The binomial distribution is defined by two parameters:

• $n$: the number of trials (in this case, $n = 20$, since there are 20 questions).
• $p$: the probability of success (in this case, $p = \frac{1}{5}$, since each question has 5 possible answers, and only one is correct).

We are asked to find the probability of passing, which means getting at least 70% of the answers correct. This corresponds to:

• A minimum of 70% correct answers = $0.70 \times 20 = 14$ correct answers out of 20.

Thus, we need to calculate the probability of getting at least 14 correct answers out of 20, assuming random guessing. This requires summing the probabilities for getting 14, 15, 16, 17, 18, 19, or 20 correct answers.

The probability mass function (PMF) of the binomial distribution is given by:

$P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}$

where:

• $\binom{n}{k}$ is the binomial coefficient (combinations),
• $p^k$ is the probability of getting exactly $k$ correct answers,
• $(1-p)^{n-k}$ is the probability of getting $n-k$ incorrect answers.

For this problem, $n = 20$, $p = \frac{1}{5} = 0.2$, and we need the probability of $X \geq 14$.

We sum the binomial probabilities for $k = 14$ through $k = 20$:

$P(X \geq 14) = P(X = 14) + P(X = 15) + P(X = 16) + P(X = 17) + P(X = 18) + P(X = 19) + P(X = 20)$

However, calculating this by hand is quite tedious. Typically, we use technology or statistical software to calculate this probability.

Using a calculator or software, the cumulative probability $P(X \geq 14)$ is approximately 0.0003. This means that the probability of passing the quiz by guessing randomly is about 0.03%.

Summary

The probability of passing the quiz by guessing randomly (getting at least 14 out of 20 questions correct) is approximately 0.03%.

Would you like a detailed explanation of any step, or do you have further questions?

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Here are five related questions to expand this concept:

1. What is the expected number of correct answers for a random guesser in this quiz?
2. How does the probability change if the passing grade is lowered to 60%?
3. What is the probability of getting exactly 10 correct answers by guessing randomly?
4. How would the probability change if there were 4 possible answers per question instead of 5?
5. Can you calculate the variance in the number of correct answers for this quiz?

Tip: Binomial distributions are great for modeling yes/no scenarios like guessing on multiple-choice tests!