The number of long distance phone calls between two cities in a certain time period varies jointly as the populations of the cities, p1 and p2, and inversely as the distance between them. If 90,000 calls are made between two cities 450 miles apart, with populations of 120,000 and 110,000 how many calls are made between cities with populations of 50,000 and 90,000 that are 350 miles apart?

Solution:

                      N = k P1P2 /R

                       90000 = k(120000)(110000)/450

                        N = k(50000)(90000)/350

                            = (90000)(450)(50000)(90000)/[(350)(120000)(110000)

                            =  39448