Find total time taken by each person to process total number of letters.

Hi guys

The following query wors fine and give me the list of persons and corresponding to each person name, how many letters each person has done.

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Now I want to calculate the total time taken by each person to do those letters. The Date1 field stores Date and time with each record in the database when it was processed.

e.g the result of the above query is as follows:

Username Total Letters
Smith 22
Martin 15
Sarah 10

Now I want to display the output like this:

Username Total Letters Time taken
Smith 22 00:45:00
Martin 15 00:30:00
Sarah 10 00:30:00


Can anyone please tell me how can I find the total time taken by each person to do total number of letters. The table stores Date and time when each record was processed.

Thanks
Aman