Kamila buys a dress for £30 in a sale. It was advertised "25%off". Kamila says"25% of £30 is £7.50 so i saved £7.50". Explain why is wrong ,and show that she has actually saved £10.

She was wrong in her analysis because the sales discount of 25% is the discount on the original cost of the dress and not the purchasing cost of the dress as explained below:

Step-by-step explanation:

From the question:

Purchasing cost price of the dress = £30

However, this cost (£30) was after a 25% Sales Discount        

It, therefore, implies that £30 is equal to 75% of the cost price

75% of the original cost price  = £30

100% of the original cost price = (30 x100%)/75%

   Original cost price                 = £40

Therefore the 25% discount / savings on the cost

                                                     = £ (40-30)

                                                      = £10    

She saved £10 by buying the dress at £30 instead of the original cost of £40