Click on the picklist arrow in any column with numeric data and select 'Summarize this Field"

You can create one or more summaries by selected Sum, Average, Max or Min.

When you run the report you'll see the calculated sum of (in this case) the total hours, as well as a grand summary at the bottom of the report.

You can choose to 'hide details' if you just want to see the calculations.

The values of zero in this report just tell us that no hours of service were verified for those opportunities.

The number of records tells us how many occurrences are 'hidden' in the data.