Your notation is non-standard and confusing. Nevertheless, I believe you want that 2.5 units of B equals 1 unit of A. You can get there by taking
Keep in mind that the percentages you gave will always give equal amounts of A or B. So, 5% of B is the same as 2.5 units of B is the same as 5% of A is the same as 1 unit of A.
I'm only working based on the information given, which, as stated above, is presented with non-standard notation/terminology.