How do I show a percentage of a project completion?

Hi all,

So I started using Excel for project management and I am starting to get the hang of it.
However, I can't figure out this one little thing.

I have 2 rows.
One row shows the number of jobs completed
The second row shows the number of jobs not completed

Lets say there are a total of 27 jobs.
All 27 jobs being completed should equal 100% So 27 = 100%

I want to make a third row showing that percentage, which should be based off of the first row.

Example:

Jobs Completed: 15
Job Not Completed: 12

Percent Completed: ---

Now just to let you know, I have tried some formula's and have wound up with the reverse percentage (i.e. The percentage incomplete) and even answers such as "1200%"
This seems like it should be easy, but I'm stumped.

Can anyone assist?