I think that inches would be most common for height but centimeters and feet could also be used.
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.