A tradesman sold an article at a loss of 20%. If the selling price had been increased by $100, there would have been a gain of 5%. What was the cost price of the article ? Options.
A) $ 100  B) $ 200  C) $ 300  D) $ 400  E) $ 500

Rationale: "let c . p . be $ x then , ( 105 % of x ) - ( 80 % of x ) = 100 or 25 % of x = 100 x / 4 = 100 x = 400 c . p . = $ 400 correct option is d"
Answer: D