A=average cost of one item, B=average cost of other item. Total cost=20A+10B.
Sale revenue of A item=20*1.16A=23.2A; revenue from B=10*0.96B=9.6B.
Profit/loss in real terms=Revenue-cost=3.2A-0.4B.
Profit/loss=(3.2A-0.4B)/(20A+10B) as a fraction of cost.
Profit/loss as percentage=100(3.2A-0.4B)/(20A+10B)=(320A-40B)/(20A+10B)=
(32A-4B)/(2A+B)% or 4(8A-B)/(2A+B)%.
If A=B (cost of all articles is the same): 28A/3A=9.33%