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.