Easiest way to work it out is to divide by ten, that gives 10% of your original number. Multiply that by two to get 20% and minus that from the original number to get the discount.
450/10 = 45 which is 10% of 450.
20% of 450= 10% x 2 or 45 x 2
therefor 20% of 450 is 90.
450-90 = the discount the shoppers recieve.
