First, enter your labels:
In cell A1, type: Total
In cell A2, type: Attend
In cell A3, type: Absent
Then, enter the data
In cell B1, type: 20
In cell B2, type: 11
In cell B3, type: =B1-B2
Select cells A2:B3
Click the Chart Wizard button, on the toolbar, and create a pie chart.