How to work a year on year figure
 

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.