To display minutes and seconds in the format mm:ss:

1. Select the cells containing the minutes and seconds data.
2. Right-click and select "Format Cells".
3. In the "Number" tab, select "Custom" from the Category list.
4. In the "Type" field, enter "mm:ss" and click "OK".

To calculate the average of the minutes and seconds data:

1. Select the cell where you want to display the average.
2. Type "
   Formula:

   =AVERAGE( 

   " and then select the cells containing the minutes and seconds data.
3. Type "
   Formula:

   ) 

   " and press Enter.

This will calculate the average of the minutes and seconds data and display it in the selected cell. To format the result as mm:ss, you can apply the custom number format as described above.