Question

A store, on average, has 500 customers per day.

a) what can be said about the probability that it will have at least 700 customers on a given day?
from now on, suppose in addition that the variance of the numbers of customers per day is 100.
b) what can be said about the probability that it will have at least 700 customers on a given day?
c) what can be said about the probability that there will be more than 475 and less than 525 customers on a given day?

Answer 1

a) We can not estimate the probability.

b) Zero probability.

c) There is a probability between 95% and 99% that they have between 475 and 525 customers on a given day.

Explanation:

a) We can not said nothing because we only know the average of customers per day. We need to know the probability distribution of the amount of customers per day to answer this question.

b) Now that we know that the variance is 100, although we do not know the exact distribution of the values, we can use the empirical rules to estimate the probability of having at least 700 customers on a given day.

If the variance is 100, the standard deviation is √100=10.

Applying the empirical rule (68-95-99.7 rule), we know that there is probability 0.15% of having at least 500+3*10=530 customers per day (more than 3 deviations from the mean).

Then, we can conclude that the probability of having at least 700 customers per day is zero.

c) To estimate this probability, we have to calculate how many deviations from the mean this values represent:

`\Delta_1=475-500=-25=2.5\sigma\\\\\Delta_2=525-500=25=2.5\sigma`

We have an interval that have a width of ±2.5 deviations from the mean.

For 2 deviations from the mean, it is expected to have 95% of the data.

For 3 deviations from the mean, it is expected to have 99.7% of the data.

Then, for the interval 475 to 525, we can estimate a probability between 95% and 99%.