Thanks your discussion was really helpful and I found solution by this
Name=last
field to summarize=column b field
type= sum
evaluate = use a formula
if onLastRecord then true else next(descfield) <> descfield
reset=on change of group
Instead of
Name=last
field to summarize=column b field
type= sum
evaluate = use a formula
next(descfield) <> descfield or onlastrecord
reset=on change of group
IP Logged