
Concept explainers
A multiple-choice exam has 100 questions. Each question has four choices, (a) What minimum score should be required to reduce the chance of passing by random guessing to 5 percent? (b) To 1 percent? (c) Find the
a.

Find the minimum score should be required to reduce the chance of passing by random guessing to 5%.
Answer to Problem 107CE
The minimum score should be required to reduce the chance of passing by random guessing to 5% is 32.12.
Explanation of Solution
Calculation:
It is given that a multiple-choice exam has 100 questions, with 4 options each.
Assume that the random variable X defines the number of correct answers by a student randomly guessing the answers. Consider choosing a correct answer as a success. Then, X has a binomial distribution.
Only 1 out of the 4 options is correct for each question. Thus, the probability of success is
Normal distribution:
A continuous random variable X is said to follow normal distribution if the probability density function of X is,
Binomial distribution:
A discrete random variable X is said to follow binomial distribution if the probability mass function is defined as,
It is known that, when
Here,
Hence,
As the conditions for approximation are satisfied, the normal approximation can be used.
The expected number of the random variable X is,
The standard deviation of the random variable X is,
The approximate normal probability that a student passes by random guessing is 5%.
Denote
Again, the probability
Variable value:
Software procedure:
Step-by-step software procedure to obtain variable value using EXCEL is as follows:
- • Open an EXCEL file.
- • In cell E1, enter the formula “=NORM.INV(0.95,25,4.33)”.
- Output using EXCEL software is given below:
Hence,
Therefore,
Thus, the minimum score should be required to reduce the chance of passing by random guessing to 5% is 32.12.
b.

Find the minimum score should be required to reduce the chance of passing by random guessing to 1%.
Answer to Problem 107CE
The minimum score should be required to reduce the chance of passing by random guessing to 1% is 35.07.
Explanation of Solution
Calculation:
Denote
Again, the probability
Variable value:
Software procedure:
Step-by-step software procedure to obtain variable value using EXCEL is as follows:
- • Open an EXCEL file.
- • In cell E1, enter the formula “=NORM.INV(0.99,25,4.33)”.
- Output using EXCEL software is given below:
Hence,
Therefore,
Thus, the minimum score should be required to reduce the chance of passing by random guessing to 1% is 35.07.
c.

Find the quartiles for a student who guesses.
Answer to Problem 107CE
The first and third quartiles of scores for a student who guesses are respectively 22.08 and 27.92.
Explanation of Solution
Calculation:
The first quartile of a distribution is the value of the variable, below which, 25% of the observations lie. In other words, the probability that an observation lies below the first quartile is 0.25.
Denote
First quartile:
Software procedure:
Step-by-step software procedure to obtain the first quartile using EXCEL is as follows:
- • Open an EXCEL file.
- • In cell E1, enter the formula “=NORM.INV(0.25,25,4.33)”.
- Output using EXCEL software is given below:
Here,
Hence,
The third quartile of a distribution is the value of the variable, below which, 75% of the observations lie. In other words, the probability that an observation lies below the third quartile is 0.75.
Denote
Third quartile:
Software procedure:
Step-by-step software procedure to obtain the third quartile using EXCEL is as follows:
- • Open an EXCEL file.
- • In cell E1, enter the formula “=NORM.INV(0.75,25,4.33)”.
- Output using EXCEL software is given below:
Here,
Hence,
Thus, the first and third quartiles of scores for a student who guesses are respectively 22.08 and 27.92.
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Chapter 7 Solutions
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