If you select the Calculate tab of the Field Properties dialog for a text field, you will see where you can configure the calucation using the Average option. For the first problem, you can use the simplified field notation option and specify:
(G8 + G9 + G10 + G11 + G12) / 20
Where G8...G12 are the field names. Replace this with the actual field names and don't use any spaces or punctuation for the field names.
There is even an option to "sum" and"average" fields by check selection. Use the first option "The field is the ____ of fields:". You may need to click around the selection box before the individual fields will check. The interface does not line up very well.