Cake Decorating

Campervan Cake

I have to make a cake in the shape of a campervan to serve 60 people for a birthday on 24th September.  I plan to make a 30cm (12 inch) square cake, cut it in half (so it measures 15cm (6 inches), sandwich with buttercream and cover it with the same, before finishing with fondant.  If each of the 15cm slices are cut into 2.5x2.5cm portions, can anyone please tell me if there will be at least 60 servings?
Many thanks!