$$James$$ sold $$1680$$ cookies from a box of cookies in the first month and kept the rest of the cookies. $$James$$ sold~$\tfrac{7}{20}$~more cookies from another similar box of cookies in the second month and kept the rest of the cookies. If the number of cookies $$James$$ kept decreased by~$\tfrac{3}{20}$, how many cookies were there in the box? 
Overseas Competition->Knowledge Point->Number Theory Modules->Prime Factorization->Applying Prime Factorization->Equalizing products by Grouping