Using the formula (b) and replacing the given values:
Sale Price = Original Price - Amount Saved. So,
Sale Price = 1/5 - 0.18
Sale Price = £0.02 (answer).
This means the cost of the item to you is £0.02.
You will pay £0.02 for an item with an original price of £1/5 when discounted 90%.
In this example, if you buy an item at £1/5 with 90% discount, you will pay 0.2 - 0 = 0.02 pounds.