The answer is correct. 1 represents 100% which is exactly the increase of 40 to 80.

Looking at it logically - if 10% of 40 is 4, then the resultant is 44
20% of 40 is 8, the resultant sum or increase from 40 is 48
50% of 40 is 20, the resultant sum or increase from 40 is thus 40 + 20 = 60
100% of 40 is 40, the resultant sum or increase from 40 is thus 40 + 40 = 80


I use this formula all the time in my calculations ot determine % increases or decreases. To have it display as a %, just set the cell or column format to % and also set the decimal places you would like to see

Another way to do this is (End Value - Begin Value) / Begin Value x 100

Which would be (A1-B1)/A1*100