Final answer:
To calculate the percentage change, subtract the old value from the new value, divide it by the old value, and multiply by 100%.
Step-by-step explanation:
To find the percentage change, we can use the formula:
Percentage change = (New value - Old value) / Old value * 100%
In this case, the old value is £80 and the new value is £100.
Substituting these values into the formula, we get:
Percentage change = (£100 - £80) / £80 * 100%
Simplifying, we get:
Percentage change = £20 / £80 * 100%
This can be further simplified to:
Percentage change = 0.25 * 100%
Therefore, the percentage change is 25%.