Use shared variable formula or running totals to achieve this.
try this
in the Field Explorer
right click on Running Total Fields
select New
Name it (e.g. "Avg 2880 at group")
field to summarize=table.SLA
type of summary = average
evaluate=use a formula
{table.record_ID <> previous({table.record_ID}) and {table.SLA} > 2880
reset= on change of group-select @assigend state
place this RT field in group footer
RTs do not work in headers