I know this years sales and the % change over Last Year ( it is down) how do I find Last Years sales?

Start with the formula for percentage change from one year to the next. If we let x_0 be the initial value, and x_1 be the value one year later, then the change in sales expressed as a percent of the initial value is then:


p =\ \frac {\Delta x} {x_0} \times 100 \ =\ \frac {x_1 - x_0} {x_0} \times 100


So if you know the percent change p and the value for x_1 you can rearrange this to get:


x_0 = \frac {x_1}{(1 + \frac p {100})}


For example: if x_1=1000 and p = 33.3% then


x_0 = \frac {1000}{(1 + \frac {33.3} {100})} = \frac {1000}{1.333} = 750


You can check this by substituting this value for x_0 back into the first equation and see if it agrees:


p = \frac {x_1 - x_0} {x_0} \times 100 = \frac {1000-750} {750} \times 100 = 33.3%