Multiplying Values between 2 tables with Relationship many to one

Do you want a calculation per row?  if so you need both a measure and a new column in your quantity sold table.

The value you get when you multiply the sum's is the total days X total sold which is a single very large number.  But you just simply use it as the data source to a new column and you get the info you want per line item.

I used very simple data but it looks like this:

my total multiplied is the same as your sum formula but gives me 72K (780x93) but I am guessing you want it by row so you can see product 1 ran for 31 days and sold 5 - multiplied together gives you 155?

Or were you wanting some other result like days per unit sold etc?