"Don Guillett" wrote in message
...
=IF(J2="","",IF(L2="",G2,G2/2))
=IF(L2="","",G2/2)
in the summary
=SUMIF('New Cars'!J:J,A4,'New Cars'!K:K)+SUMIF('New Cars'!L:L,A4,'New
Cars'!M:M)

Why such complication? KISS!!!! If the OP needs to do this, then just have
a subsidiary sheet with the details on it and summarize it on the front
sheet.
As a recently-retired Management Accountant I can guarantee you that
some-one is going to say "give me an analysis of split sales". Why do the
work twice?