Hey barca.

You will need to first create a cumulative distribution for sales and identify the stores associated with each point.

Once you have the distribution, you get the point x where P(X < x) = 0.15 and all stores in that region will be in the bottom 50% range.

To do this get your sales data and build a histogram of it and use a statistical package to find what sales value the bottom 15% (or other percentage) is in.