The approaches that I thought are
1. # of people who use convenient store in a week / # of customers of one convenient store in a week
2. # of Tokyo's convenient stores in 1 ㎢ * 2000 ㎢(the area of Tokyo) * (the area of Japan / the area of Tokyo) (PS: Assume that # of convenient stores are in proportion to the population)
Besides these two approaches, I wonder if there is any other approach to solve this problem.