Nawk user-defined function

HELP!!!! I am in an on-line shell programming class and have a question. Here is the data:

Mike Harrington:(510) 548-1278:250:100:175
Christian Dobbins:(408) 538-2358:155:90:201
Susan Dalsass:(206) 654-6279:250:60:50
(There are 12 contribuors total)

This database contains names, phone numbers and money contributions to a party campaigh for the past three months.

How do I write a user-defined function to return the average of all the contributions for a given month? The month will be passed in at the command line.

Thanks!!

We have a few rules here. In particular,