Question

A study of peach trees found that the average number of peaches per tree was 725. The standard deviation of the population is 70 peaches per tree. A scientist wishes to find the 95% confidence interval for the mean number of peaches per tree. How many trees does she need to sample to obtain an average accurate to within 10 peaches per tree

Answer 1

Answer:

She needs to sample 189 trees.

Step-by-step explanation:

We have that to find our $\alpha$ level, that is the subtraction of 1 by the confidence interval divided by 2. So:

$\alpha = \frac{1 - 0.95}{2} = 0.025$

Now, we have to find z in the Ztable as such z has a pvalue of $1 - \alpha$.

That is z with a pvalue of $1 - 0.025 = 0.975$, so Z = 1.96.

Now, find the margin of error M as such

$M = z\frac{\sigma}{\sqrt{n}}$

In which $\sigma$ is the standard deviation of the population and n is the size of the sample.

The standard deviation of the population is 70 peaches per tree.

This means that $\sigma = 70$

How many trees does she need to sample to obtain an average accurate to within 10 peaches per tree?

She needs to sample n trees.

n is found when M = 10. So

$M = z\frac{\sigma}{\sqrt{n}}$

$10 = 1.96\frac{70}{\sqrt{n}}$

$10\sqrt{n} = 1.96*70$

Dividing both sides by 10:

$\sqrt{n} = 1.96*7$

$(\sqrt{n})^2 = (1.96*7)^2$

$n = 188.2$

Rounding up:

She needs to sample 189 trees.