Coffee shops are planned to be placed in an office building. The office tenants are located at P 1 (20, 70), P2 (30, 40), P3 (90, 30) and P4 (50,100). 100 persons per day are expected to visit the first office, 60 in the second office, 140 in the third office and 120 in the last office. 70% ofvisitors are expected to drop by the coffee shop. Each unit distance which a customer has to travel costs the owner of the coffee shops the loss of $0.25 in revenue. The daily operating cost of n shops is $5000n. Determine the number of coffee shop and their locations.