Final answer:
The overall decrease in the price of the dress is 1/3 of the original price.
Step-by-step explanation:
To find the overall decrease in the price of the dress, we need to calculate the original price of the dress.
Let's assume the original price of the dress is 'x'.
The normal price of the dress is reduced by 25%, which means it is now priced at 75% of the original price, or 0.75x.
The sale price is then reduced by £10, so we have the equation: 0.75x - £10 = £80.
Now, we can solve for 'x'. Add £10 to both sides of the equation: 0.75x = £90. Divide both sides by 0.75: x = £120.
Therefore, the overall decrease in the price of the dress is £120 - £80 = £40. As a fraction of the original price (£120), the overall decrease is 40/120 = 1/3.