how do I do this

[vuthlarhi donald]

Write a Math Utility that takes a Collection of any number (Integer) and
returns the average of that collection.

[Rob Spoor]

Calculate the sum over the elements and then divide by the count.
Keep in mind though what to do with null values - you'll probably want to exclude them completely, and therefore you'll have to calculate the count as well - at the same time as the sum.