# Thread: Help with a normal distribution question

1. ## Help with a normal distribution question

The average amount spent by customers on a single visit to a particular supermarket is £42 with standard deviation £24. In addition, the amounts spent have a normal distribution.

Giving your answers to 2 decimal places, use Excel to find the following:

(a) What percentage of the supermarket's customers spend
in excess of £45 in a single visit?

(b) What percentage of the supermarket's customers spend
less than £25 in a single visit?

(c) What percentage of the supermarket's customers spend
between £35 and £55?

(d) The supermarket wishes to introduce a minimum amount
for which credit cards may be used which enables 85% of customers
to pay by credit cards. At what amount should this credit card
minimum be set?

I'm unsure how to answer this question any help would be really appreciated. Thanks

2. ## Re: Help with a normal distribution question

You need to use a table of normal distribution values to evaluate these.

First you need to change to the standard normal distribution: if x has mean $\displaystyle \mu$ and standard deviation $\displaystyle \sigma$ then the $\displaystyle z= \frac{x- \mu}{\sigma}$ has the standard normal distribution. If you are asked a problem like this, I suspect you have already been taught this and that your text has a table of the standard normal distribution in the back. If not there is one here: Standard Normal Distribution Table.

3. ## Re: Help with a normal distribution question

Originally Posted by HallsofIvy
You need to use a table of normal distribution values to evaluate these.

First you need to change to the standard normal distribution: if x has mean $\displaystyle \mu$ and standard deviation $\displaystyle \sigma$ then the $\displaystyle z= \frac{x- \mu}{\sigma}$ has the standard normal distribution. If you are asked a problem like this, I suspect you have already been taught this and that your text has a table of the standard normal distribution in the back. If not there is one here: Standard Normal Distribution Table.
Do you know how to do it in excel?

4. ## Re: Help with a normal distribution question

Originally Posted by HallsofIvy
You need to use a table of normal distribution values to evaluate these.

First you need to change to the standard normal distribution: if x has mean $\displaystyle \mu$ and standard deviation $\displaystyle \sigma$ then the $\displaystyle z= \frac{x- \mu}{\sigma}$ has the standard normal distribution. If you are asked a problem like this, I suspect you have already been taught this and that your text has a table of the standard normal distribution in the back. If not there is one here: Standard Normal Distribution Table.
Would you mind going through the answers with me? I would be so grateful.