Using SUM To Give Interim Totals Every 'x' Rows From A Query

First off, here is the application.

An airport baggage handling system distributes bags using multiple conveyors.
Bag counts are logged every 15 minutes. There is a count for each conveyor.
Example Log Table layout is as follows (The TIME column is DateTime, the Convx columns are TinyInt)

Time Conv1 Conv2 Conv3 Conv4 Conv5 Conv6

i 3 2 3 4 2 1

i+15min 2 3 4 2 2 2

i+30min etc.....................

i+45min

i+60min

etc...



The management team wants a throughput report which will take the following parameters in order to filter the results:

Begin Date
End Date
Time Interval (selectable as 15mins, 30 mins, 45mins, 60 mins and Daily)

My question is this. Given that my raw data has 1 row for every 15 minutes, if they select 60 minutes as their interval I need to run the query with the start and end dates but Sum every 4 rows and display it as 1 row, likewise if they select 30 minute interval, I need to sum every 2 rows. How do I run a query and SUM the Conv count data for every x number of rows and use the 1st TIME value in the returned x row summary?



Thanks for your help and let me know if I need to clarify anything