InfoPath Dev

I am developing Infopath 2010 list form. There is a drop-down list field (e.g. let's call it "EstimatedTimeSpent") It stores time increment of 5 minutes value. e.g. 00:05, 00:10, 00:15 and so on. There is a Read-only Text box (e.g. let's call it "TotalTimeSpent") There is a need to store this value and when next time user edits this form, this value will be added to current user selection from the drop-down list. For instance: If the user picked 00:15 (15 minutes) the first time then when user revisits this form or edit this form, picks 00:25 (25 minutes) there need to be a field which holds this summation. Rules/Formula needed: TotalTimeSpent = EstimatedTimeSpent (Previously submitted value) + EstimatedTimeSpent (Current user selection)

I was able to resolve this using Multiple checklist value