apply discount based on number of guests

I am trying to make a spreadsheet to calculate prices for a customer, the problem I am having is applying discounts based on the amount that they purchase. For example;

A1 is # of customers A2 is price per unit
B2 is sub total

This is easy enough, my problem is I want to make a rule for B2 that states "If the # in A1 is greater than 20 than take off 1xA2"

How is this done?
Thanks for the help folks!!!

Ok, thank you for your help...I guess because there's more lines it complicates it further, so here is what I have,

A1 is # of customers A2 is price per unit A3 is Subtotal for line A
B2 is # of customers B2 is (different) PPU B3 is Subtotal for line B
C3 is =A3+B3

Now what I want, is for C3 to reflect, that if (A1+B1)>20 than C3 should minus 1*A2, but this should also include the total of A3+B3.

Sorry if that's worded confusingly...i'm trying my best here....

Thank you, That did work, but it only when the values in A1+B1 were over 20, if they were 20 or under it equaled whatever the added # of A1+B1 was.

I would be happy to provide a sample file. but I am unsure how to do so. If you could explain or provide a link to the explanation

To Attach a File:

1. Click on Go Advanced
2. In the frame Attach Files you will see the button Manage Attachments
3. Click the button.
4. A new window will open titled Manage Attachments - Excel Forum.
5. Click the Browse... button to locate your file for uploading.
6. This will open a new window File Upload.
7. Once you have located the file to upload click the Open button. This window will close.
8. You are now back in the Manage Attachments - Excel Forum window.
9. Click the Upload button and wait until the file has uploaded.
10. Close the window and then click Submit.

That last equation ended up giving me a wrong price when A1+A2 were less than 20.

I have included the sample on this post.

THANK YOU!!! great job!