Please help with understanding of Aggregate calculations for host group

[Paul_]

Hello.
Could you please help with understanding of Aggregate calculations for host group.
If I have items for each host: sys.a, sys.b. And host group HostGRP with hosts.

1. How to calculate a new item “GRP_sum_div_10” like: sum( last(sys.a) * last(sys.b) for all from HostGRP) / 10 ?
2. Where is to create this new item GRP_sum_div_10? If I create this item in host1, Zabbix will show graph with legend “host1: GRP_sum_div_10”. But need GRP_sum_div_10 (without host1).

Like fast solution I created a new item per host with result of sys.a*sys.b/10 and created item with Aggregate calculations sum of host results from HostGRP.
But is there possible to do it in one item by Aggregate calculations?

Thak you!