x = original price or 100%
so if we discount "x" by 12%, that means the new price is 100% - 12% = 88%, let's see how much is 88% of "x"
now if we grab that new price, or the new 100%, and discount it further by 5%, what's leftover is simply 95%, because we'll be taking off 5% off of it and thus left with 100% - 5% = 95%.
so what's 95% of 0.88x?
so the new discounted price is just 0.836x or 83.6% of our original "x".
from 100% originally to 83.6%, the difference is 16.4%, and that's the total discount.