Respuesta :
Answer:
0.0016 probability that greater than 55 (exclusive) but less than 75 (exclusive) sales will be made.
Step-by-step explanation:
We use the normal approximation to the binomial distribution to solve this question.
Binomial probability distribution
Probability of exactly x sucesses on n repeated trials, with p probability.
Can be approximated to a normal distribution, using the expected value and the standard deviation.
The expected value of the binomial distribution is:
[tex]E(X) = np[/tex]
The standard deviation of the binomial distribution is:
[tex]\sqrt{V(X)} = \sqrt{np(1-p)}[/tex]
Normal probability distribution
Problems of normally distributed distributions can be solved using the z-score formula.
In a set with mean [tex]\mu[/tex] and standard deviation [tex]\sigma[/tex], the zscore of a measure X is given by:
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
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.
When we are approximating a binomial distribution to a normal one, we have that [tex]\mu = E(X)[/tex], [tex]\sigma = \sqrt{V(X)}[/tex].
A company's sales force makes 400 sales calls, with 0.25 probability that a sale will be made on a call.
This means that [tex]n = 400, p = 0.25[/tex]
Mean and standard deviation:
[tex]\mu = E(X) = np = 400*0.25 = 100[/tex]
[tex]\sigma = \sqrt{V(X)} = \sqrt{np(1-p)} = \sqrt{400*0.25*0.75} = \sqrt{75}[/tex]
What is the probability that greater than 55 (exclusive) but less than 75 (exclusive) sales will be made?
Using continuity correction, this is [tex]P(55+0.5 \leq X \leq 75-0.5) = P(55.5 \leq X \leq 74.5)[/tex], which is the p-value of Z when X = 74.5 subtracted by the p-value of Z when X = 55.5.
X = 74.5
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{74.5 - 100}{\sqrt{25}}[/tex]
[tex]Z = -2.94[/tex]
[tex]Z = -2.94[/tex] has a p-value of 0.0016
X = 55.5
[tex]Z = \frac{X - \mu}{\sigma}[/tex]
[tex]Z = \frac{55.5 - 100}{\sqrt{25}}[/tex]
[tex]Z = -5.14[/tex]
[tex]Z = -5.14[/tex] has a p-value of 0
0.0016 - 0 = 0.0016
0.0016 probability that greater than 55 (exclusive) but less than 75 (exclusive) sales will be made.
