Need help -- running total of hours worked on a project

Need help -- running total of hours worked on a project

I've created a DB that allows you to assign a task/project to others. It also allows the person to keep updating the task/project showing how many hours that have worked.

How can I keep a running total of hours per task/project?
Example:

Task1
I just worked 5 hours
Previously worked hours: 2
Total hrs field should = 7

Any help would be appreciated!

--Sid