Mark-Up
Mark-up is difference in retail price and the cost price, that is
R = C + M
Where R = retail price
C = cost price
M = mark-up


Markdown
Mark down is decrease in the selling price. It is the difference in the old retail price and the new retail; that is ,
MD = OP - NP
Where MD = markdown
OP = old retail price
NP = new retail price