Rule:
When an item is marked UP n%, its new price is (100+n)% of the old price.
When an item is marked DOWN n%, its new price is (100-n)% of the old price.

When the item is marked up 20%, its new price is (100+20)% or 120% of the
original price.  That amounts to multiplying by 1.2.

Then when the item is marked down 15%, its new price is (100-15)% or 85% 
of the increased price.  That amounts to multiplying by 0.85.

So here we have in effect multiplied the original price once by 1.2 then again
by .85, which amounts to having multiplied the original price by 1.2×.85 = 1.02,
which is 102% of the original price. 

Edwin