Hey noise, sorry for not being clear enough, my English is a bit limited (even more when is a specific topic such as this)
There are two ways of adding discounts that I know of, one is by stacking two percentages:
20% + 20% = 40%;
And the other is by multiplying the product of the discounts:
(1-20%)*(1-20%) = 64%
discount = 1 - 0.64 = 0.36
discount = 36%
The second one is the most commom case and will be used 90% of the times. it is the one I was referring to. But I don’t know if in English you would use another term rather than multiply the discounts, I think that the term for this would be “Compound Interest” while the other one I think that is called as “Simple Interest”.