Heights of 10-year-olds, regardless of gender, closely follow a normal distribution with mean 55 inches
and standard deviation 6 inches.
a. What is the probability that a randomly chosen 10-year-old is shorter than 48 inches?
b. What is the probability that a randomly chosen 10-year-old is between 60 and 65 inches?
c. If the tallest 10% of the class is considered “very tall”, what is the height cutoff for “very tall”?
d. The height requirement for Batman the Ride at Six Flags Magic Mountain is 54 inches. What
percent of 10-year-olds cannot go on this ride?
Answers
Answer:
a) 12.10% probability that a randomly chosen 10 year old is shorter than 48 inches.
b) 15.58% probability that a randomly chosen 10 year old is between 60 and 65 inches.
c) The cutoff for very tall is 62.68 inches.
d) 43.25% of 10 year olds cannot go on this ride.
Step-by-step explanation
Problems of normally distributed samples can be solved using the z-score formula.
In a set with mean and standard deviation , the zscore of a measure X is given by:
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
In this problem, we have that:
(a) What is the probability that a randomly chosen 10 year old is shorter than 48 inches?
This is the pvalue of Z when X = 48. So:
has a pvalue of 0.1210.
So there is a 12.10% probability that a randomly chosen 10 year old is shorter than 48 inches.
(b) What is the probability that a randomly chosen 10 year old is between 60 and 65 inches?
This is the value of Z when X = 65 subtracted by the pvalue of Z when X = 60.
X = 65
has a pvalue 0.9525
X = 60
has a pvalue 0.7967
So there is a 0.9525-0.7967 = 0.1558 = 15.58% probability that a randomly chosen 10 year old is between 60 and 65 inches.
(c) If the tallest 10% of the class is considered "very tall", what is the height cutoff for "very tall"?
This is the value of X when Z has a pvalue of 0.9. So it is X when
The cutoff for very tall is 62.68 inches.
(d) The height requirement for Batman the Ride at Six Flags Magic Mountain is 54 inches. What percent of 10 year olds cannot go on this ride?
This is the pvalue of Z when X = 54.
has a pvalue of 0.4325.
So 43.25% of 10 year olds cannot go on this ride.
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