Cherry trees in a certain orchard have heights that are normally distributed with mean U = 112 inches and standard deviation O = 10 inches.

(a) Find the 22nd percentile of the tree heights.

(b) Find the 80th percentile of the tree heights.

(c) Find the first quartile of the tree heights.

(d) An agricultural scientist wants to study the tallest 1% of the trees to determine whether they have a certain gene that allows them to grow taller. To do this, she needs to study all the trees above a certain height. What height is this?

User Icon for Bot GPT 4-o mini Bot GPT 4-o mini answered
2 hours ago

To solve these problems, we can use the properties of the normal distribution along with the z-score formula. The z-score formula is given by:

\[ z = \frac{(X - \mu)}{\sigma} \]

where \(X\) is the value of interest, \(\mu\) is the mean, and \(\sigma\) is the standard deviation.

Given:

  • Mean (\(\mu\)) = 112 inches
  • Standard deviation (\(\sigma\)) = 10 inches

We will use the standard normal distribution table (z-table) or a calculator to find the z-scores corresponding to the percentiles.

(a) Find the 22nd percentile of the tree heights.

The z-score corresponding to the 22nd percentile (0.22) can be found using a z-table or calculator, which gives a z-score of approximately -0.77.

Now we use the z-score formula to find \(X\):

\[ -0.77 = \frac{(X - 112)}{10} \]

Solving for \(X\):

\[ X - 112 = -0.77 \times 10 \] \[ X - 112 = -7.7 \] \[ X = 112 - 7.7 = 104.3 \text{ inches} \]

(b) Find the 80th percentile of the tree heights.

The z-score corresponding to the 80th percentile (0.80) is approximately 0.84.

Now we use the z-score formula to find \(X\):

\[ 0.84 = \frac{(X - 112)}{10} \]

Solving for \(X\):

\[ X - 112 = 0.84 \times 10 \] \[ X - 112 = 8.4 \] \[ X = 112 + 8.4 = 120.4 \text{ inches} \]

(c) Find the first quartile of the tree heights.

The first quartile (Q1) corresponds to the 25th percentile (0.25). The z-score for the 25th percentile is approximately -0.67.

Now we use the z-score formula to find \(X\):

\[ -0.67 = \frac{(X - 112)}{10} \]

Solving for \(X\):

\[ X - 112 = -0.67 \times 10 \] \[ X - 112 = -6.7 \] \[ X = 112 - 6.7 = 105.3 \text{ inches} \]

(d) Height of the tallest 1% of the trees.

The tallest 1% corresponds to the 99th percentile (0.99). The z-score for the 99th percentile is approximately 2.33.

Now we use the z-score formula to find \(X\):

\[ 2.33 = \frac{(X - 112)}{10} \]

Solving for \(X\):

\[ X - 112 = 2.33 \times 10 \] \[ X - 112 = 23.3 \] \[ X = 112 + 23.3 = 135.3 \text{ inches} \]

Summary of results:

(a) 22nd percentile: 104.3 inches
(b) 80th percentile: 120.4 inches
(c) First quartile: 105.3 inches
(d) Height for the tallest 1%: 135.3 inches