Calculate A Variable Average In SQL Server?

ok so i have this table:


Code:


Products_Sold
priceSold - money
itemsSold - int


An example of 4 rows on my table would be like this
$1400 80
$1500 85
$1560 82
$1700 81

to calculate the average of the price sold related to the number of sold items just have to do
Select avg(priceSold*itemsSold)

But sometimes i just want the average price of the first 100 sold items, so how can i make my query to just use the first 100 sold items?

in math it would be like this
average= ( (1400*80) + (1500*20) ) / 100

but if i wanted the first 200 it would be like this
average= ( (1400*80) + (1500*85) + (1560*35)) / 200

and if i wanted the first 300 would be like this
average= ( (1400*80) + (1500*85) + (1560*82) + (1700*53)) / 300

but of course the number i want will always be a variable which is less than the total of the products sold. So, how the heck do i program this query where the number of the items sold is variable and it will take the rows of the database depending on how many items were sold.

I hope i didnt wrote my explanation too confusing and that i can get any help from you guys. thank you a lot for the help and byye