Mathematical question on access 2013

hi
i have in excel a formula to get a discount price for products, and every product has his  discount price (usually a percentage discount),so i have 4 column on the spreadsheet that contain the new prices, a1 the product name, a2 the original price, a3 the discount percent, a4 the new price
for example:
(product) a1 ipad.
(b1) $200.
(Discount percent) c1 0.25.
( new price is)d1  =B1/(1+C1)*1 (result 160).

 in the next row i have another product that i want to give a discount by price not by percent so i need to convert the price to percent, so the spreadsheet data will be Consistent.
for this example lets say that the product price is 400 and i want  to sell it for 360 dollars, that  is a discount of 40 dollars, how i know what is the percent of 40 dollars on 400 dollars (i know it's something like 11.11 but i want to write a formula that will give me the exact result.expert-exchange.xls