# Help with a normal distribution question

• Nov 17th 2012, 10:41 AM
Ceidon
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
• Nov 17th 2012, 10:57 AM
HallsofIvy
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 $\mu$ and standard deviation $\sigma$ then the $z= \frac{x- \mu}{\sigma}$ 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.
• Nov 17th 2012, 10:58 AM
Ceidon
Re: Help with a normal distribution question
Quote:

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 $\mu$ and standard deviation $\sigma$ then the $z= \frac{x- \mu}{\sigma}$ 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?
• Nov 17th 2012, 11:41 AM
Ceidon
Re: Help with a normal distribution question
Quote:

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 $\mu$ and standard deviation $\sigma$ then the $z= \frac{x- \mu}{\sigma}$ 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.