How to create a field in a table that is the sum of 2 other numeric fields in the same table row

How to create a field in a table that is the sum of 2 other numeric fields in the same table row

I have a table of time spent on a task in a project.  Each row of the table is a task and has a billable minutes column and a non billable minutes column.  So the table would look like this:

Task                  Billable Minutes      Non Billable Minutes
Task Name 1      25                              10
Task Name 2      12                              60     
Task Name 3      6                                15      

I want to create a column in the table that has the billable minutes and non billable minutes added together and divided by 60 for each table row.  I added a custom formula with the formula column name of Task Time.  I entered a formula ("Billable Minutes" + "Non Billable Minutes")/60.  When I click ok the column is added, but no data is showing in the column.  If I create a report using that field, the value doesn't appear on the detail lines or sub total lines, but it does appear as a grand total.  What am I doing wrong?