# Normal distribution - practice problems - page 2 of 3

It is used for random variables that are the sum of a large number of independent or only weakly dependent values. The probability distribution has a bell-shaped Gaussian curve. It is symmetric around the mean value μ, both median and mode. Three sigma rule (sigma σ = standard deviation): 68.26% of the probability belongs to the mean value μ to the distance σ, 95.45% belongs to 2σ, 99.73% to 3σ.Direction: Solve each problem carefully and show your solution in each item.

#### Number of problems found: 53

- Quartile statistics

Use the following data, which was obtained from a study of weights of adult men, to answer problems: Mean ; 192 lb ; First quartile ; 178lb Median; 185lb ; Third quartile ; 232 lb Mode ; 180 lb ; 86th percentile ; 239 lb Standard Deviation ; 23lb; ; a) Wh - Z-score test

The fish weights in a certain lake are normally distributed with a mean of 11 lb and a standard deviation of 6. If four fish are randomly selected, what is the probability that the mean weight will be between 8.6 and 14.6 lb? Your answer should be a decim - Suppose 6

Suppose the life span of a revolutionary light bulb is normally distributed with a mean life span of 70 thousand hours and a standard deviation of 3 thousand hours. If a light bulb is chosen at random: a) what is the probability the life span will be with - 10km race

Joan's finishing time for the bolder boulder 10 km race was 1.77 standard deviations faster than the women in her age group. 415 women ran in her age group. Assuming a normal distribution, how many women ran more quickly than Joan? - Z score transformation

The annual salary of an entry-level statistics major (in thousands of dollars) is normally distributed with a mean of 75 and a standard deviation of 12. X ∼ N ( μ = 75, σ = 12 ). What is the minimum salary a statistics major should aim for in order to ear - The probability

The probability that a life bulb will have a lifetime of more than 682 hours is 0.9788. The probability that a bulb will have a lifetime of more than 703 hours is 0.0051. Find the probability that a bulb will last for more than 648 hours. - Two sigma

Khail took his math test and scored 88. If the class average was 78 with a standard deviation of 5, what percent of students earned a higher grade than Khail's grade? - A biologist

A biologist knows that the average length of a leaf of a certain plant is 4 inches. The standard deviation of the population is 0.6 inches. A sample of 20 leaves of that type of plant given a new type of plant food had an average length of 4.2 inches. At - An exam - normal distribution

Five thousand students take an exam with a mean of 59 and a deviation of 8. How many students will score less than 75? - Distribution 67074

The time required to complete the test has a normal distribution with a mean of 50 minutes and a standard deviation of 10 minutes. What percentage of students take the test within 30 minutes? - Three sigma rule

Stomach weights are normally distributed, with a mean of 1314g and a standard deviation of 113g. State the probability that a randomly selected stomach weighs more than 1118g. (Report the probabilities using at least four decimal places. ) - Suppose 4

Suppose that 14% of all steel shafts produced by a certain process are nonconforming but can be reworked (rather than having to be scrapped). Consider a random sample of 200 shafts, and let X denote the number among these that are nonconforming and can be - Rich retirement

Health care issues are receiving much attention in both academic and political arenas. A sociologist recently surveyed citizens over 60 years of age whose net worth is too high to qualify for government health care but who have no private health insurance - Performance comparing

A standardized test was administered to thousands of students with a mean score of 85 and a standard deviation of 8. A random sample of 50 students was given the same test and showed an average score of 83.20. Is there evidence to show that this group has - Three sigma rule

The height of trees in a given stand is known to be a quantity with a normal probability distribution with a mean value of 15 m and a variance of 5 m². Determine the interval in which there will be tree heights in such a stand with a probability of 90% - A machine

A machine produces steel rods of normally distributed length, the mean length and the standard deviation being 50.0 cm and 0.5 cm, respectively. The rods do not conform to safety standards if they are either shorter than 49.1 cm or longer than 50.7 cm in - Statistical survey

Write TRUE OR FALSE for each question: 1 Standard deviation measures central location. 2. The most frequent observation in a data set is known as the mode. 3 The most passive method of data collection is observation. 4 Access time for secondary data is sh - A mountain climber

A mountain climber plans to buy some rope to use as a lifeline. Which of the following would be the better choice? Explain your choice. Rope A: Mean breaking strength:500lb; the standard deviation of 100lb Rope B: Mean breaking strength: 500lb; the standa - Tallest people a

As a group, the Dutch are amongst the tallest people in the world. The average Dutchman is 184 cm tall. If a normal distribution is appropriate, and the standard deviation for Dutchmen is about 8 cm, what is the percentage of Dutchmen who will be over 2 m - Statistics quiz

Fill in the missing word. 1. in a data set, the mean, median and mode are measured of ________________ 2. "The manipulation of variables under controlled conditions"is the data collection method known as______________ 3. in a normal distribution, the area

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