Per cent is a ratio. It means "per hundred".
To calculate the percentage change, you need to work out the size of the change and what percentage this was of the original value.
We're going from 100 to 140 acres. So the change is 140 - 100 = 40.
Now to work out the percentage. We do this by taking our value, dividing it by our original amount and then multiplying by 100 to give it "per cent".
40 / 100 * 100. In this example we start off with 100 which makes it nice and simple since the 100 we start with and the 100 we're dividing by to find the percentage cancel one another out.
40 / 100 * 100 = 40. So the answer is 40%
