Y2 - y1 y1100 your percentage change. Select all of the cells of the Percentage of Change column and then press CtrlD. Now divide this number by the original number.
This will only find the percentage change between a number you input and the original number.
Using the sum of this figure. First work out the difference between the two numbers 40 and 50. Divide that amount by the absolute value of the starting value. Increase New Number - Original Number.