Final answer:
To find the price before the 20% decrease, divide the discounted price by (1 - discount percentage). The original price was £525.
Step-by-step explanation:
To find the price before the 20% decrease, we can use the formula:
Original Price = Discounted Price / (1 - Discount Percentage)
Let's plug in the given values:
Original Price = £420 / (1 - 0.20)
Original Price = £420 / 0.80
Original Price = £525
Therefore, the price before the decrease was £525.