You have 2,500 square feet of selling space. you want to reserve at least 125 square feet for each product category you will carry. 30% of the space will be used for aisles. how many categories can you carry?

If 30% is devoted to aisles, 70% is devloted to merchandise for sale.

That'd come to 0.70(2500 sq ft), or 1750 sq ft.

Create a conversion factor:  125 sq ft = 1 category:
  
                                                                 1 category
                                                becomes
                                                                  125 sq ft

Muliply this conversion factor by 1750 sq ft.

Result:  1750
              categories = 14 categories.
               125

So you'd have 14 categories and will have allocated 125 sq ft to each; this comes to 1750 sq. ft, which is 70% of the total floor area.  The rest goes to aisles.