How to work a year on year figure

[Bob]

I have a sheet which works out a yearly running total on five departments of
which i can see if they are running a positive or negative growth. The data
from my workplace is previous sales volume and + or - % against last year, of
which i have to work out last years volume on a calculator(very time
consuming). This is how i do it : a1= this period £, a2= last year(same
period), a3=a1/a2-1.
What i would like to do is say this period is 100, last year is 80 then the
result would be 25% growth, all i want to do is input input 100 and the
growth of 25% and then let excel work out what last years volume was in both
positive and negative instances.

 

[Bob Phillips]

I am not sure what you need to calculate. Given this and last year sales,
you yourself state the formula.

If you want to calculate previous sales volumes, it is just =A1/(1+A3)