Hi
First off you need two tables in Access, one for name, staff id, week and total, and the other activity, time and staff IDs
Set up a relationship in Access and Infopath where for each 1 staff id there can be many staff IDs, ie a one-to-many relationship. Then,
1) there is no need to have this data in a repeating section you can just have it on the form once. In answer to your question the one-to-many relationship you setup will ensure that the data is input correctly into the database.
2) To keep it simple make them only enter the time in minutes, then right-click the repeating table and then 'Display' and tick 'footer'. Add the field "total" in the footer. Go into the "total" field and click the fx button. Click 'Insert Button' and select "Sum" click OK. double click the 'double click to insert field' and select the "time" field.
Job done, i hope.
Adam