A certain product are sold at A store, B store, and C store.
Total of 6867 pieces sold in three stores this month and the number of product increased more than the previous month was same in each store.
It means that in this month total sales of this product was increased by 15% in A store, 10% in B store and 6% in C store.
Find the sum total number of the sales of three stores in the previous month.