Question

If last year a pineapple cost​ $2, and this year it costs​ $4, there has been a​ ________ percent rise in the price of the pineapple over a​ one-year period.

Answer 1

In order to calculate the percent rise or percentage increase, we should calcualte the difference (increase) between the two numbers/prices: $4 and 2$. 
4-2=$2
Then we should divide the increase by the original number and multiply the answer by 100.
2/2*100=1*100%
So, if last year a pineapple cost​ $2, and this year it costs​ $4, there has been a​ 100 percent rise in the price of the pineapple over a​ one-year period.

Answer 2

Let us consider last year's price, $2, as the original price of a pineapple. This year, the price has risen to $4. So the increase in pineapple price is 4 - 2 = $2. To convert the price increase into a percent, follow this step: Increase in price/original cost x 100. That is, 2/2 x 100 = 100. Thus, there has been a 100 percent rise in the price of pineapple over a one year period.