Given that [tex]$z_{20} = -2$[/tex] and [tex]$z_{50} = -1$[/tex], which of the following do you know?

A. The variance is 10.
B. The standard deviation is 30.
C. The mean is 80.
D. The median is 40.
E. The data point [tex][tex]$x = 20$[/tex][/tex] is 2 standard deviations from the mean.
F. The data point [tex]$x = 50$[/tex] is 1 standard deviation from the mean.
G. The data point [tex]$x = 45$[/tex] has a [tex][tex]$z$[/tex][/tex]-value of 1.5.



Answer :

To solve the problem, we need to determine the correct information about the given data based on the z-scores provided.

Given:
- [tex]\( z_{20} = -2 \)[/tex]
- [tex]\( z_{50} = -1 \)[/tex]

### Step 1: Set up the equations for z-scores

The z-score formula is given by:
[tex]\[ z = \frac{(x - \mu)}{\sigma} \][/tex]
where [tex]\( \mu \)[/tex] is the mean and [tex]\( \sigma \)[/tex] is the standard deviation.

Using the given z-scores:
[tex]\[ z_{20} = -2 \rightarrow -2 = \frac{(20 - \mu)}{\sigma} \][/tex]
[tex]\[ z_{50} = -1 \rightarrow -1 = \frac{(50 - \mu)}{\sigma} \][/tex]

### Step 2: Solve for the mean [tex]\(\mu\)[/tex]

Rewrite the equations in terms of [tex]\(\mu\)[/tex] and [tex]\(\sigma\)[/tex]:
[tex]\[ -2 = \frac{(20 - \mu)}{\sigma} \rightarrow 20 - \mu = -2\sigma \rightarrow \mu = 20 + 2\sigma \][/tex]
[tex]\[ -1 = \frac{(50 - \mu)}{\sigma} \rightarrow 50 - \mu = -\sigma \rightarrow \mu = 50 + \sigma \][/tex]

Equating the two expressions for [tex]\(\mu\)[/tex]:
[tex]\[ 20 + 2\sigma = 50 + \sigma \][/tex]
Solving for [tex]\(\sigma\)[/tex]:
[tex]\[ 20 + 2\sigma = 50 + \sigma \][/tex]
[tex]\[ 2\sigma - \sigma = 50 - 20 \][/tex]
[tex]\[ \sigma = 30 \][/tex]

Substitute [tex]\(\sigma\)[/tex] back into one of the expressions for [tex]\(\mu\)[/tex]:
[tex]\[ \mu = 20 + 2\sigma = 20 + 2(30) = 20 + 60 = 80 \][/tex]

### Step 3: Verify the variance

The variance ([tex]\(\sigma^2\)[/tex]) is given by:
[tex]\[ \sigma^2 = 30^2 = 900 \][/tex]

### Step 4: Since the median is already given

The median is given as [tex]\(40\)[/tex].

### Step 5: Analyze the options

- "The variance is 10": This is incorrect as the variance is [tex]\(900\)[/tex].
- "The standard deviation is 30": This is correct as we calculated [tex]\(\sigma = 30\)[/tex].
- "The mean is 80": This is correct as we calculated [tex]\(\mu = 80\)[/tex].
- "The median is 40": This is correct as it is given.
- "The data point [tex]\(x = 20\)[/tex] is 2 standard deviations from the mean": This is incorrect because the deviation is correct but the calculation does not match [tex]\(z_{20}\)[/tex].
- "The data point [tex]\(x = 50\)[/tex] is 1 standard deviation from the mean": This is correct as calculated.
- "The data point [tex]\(x = 45\)[/tex] has a [tex]\(z\)[/tex]-value of 1.5": This is incorrect based on our earlier calculations.

### Conclusion

The correct answers are:
- The standard deviation is 30.
- The mean is 80.
- The median is 40.
- The data point [tex]\( x = 50 \)[/tex] is 1 standard deviation from the mean.

Thus, the valid responses are:
"The standard deviation is 30."
"The mean is 80."
"The median is 40."
"The data point [tex]\( x = 50 \)[/tex] is 1 standard deviation from the mean."