The average number of daily phone calls, C, between two cities varies jointly as the product of their populations, P₁ and P₂, and inversely as the square of the distance, d, between them.
a. Write an equation that expresses this relationship.
b. The distance between San Francisco (population: 777,000) and Los Angeles (population: 3,695,000) is 420 miles. If the average number of daily phone calls between the cities is 326,000, find the value of k to two decimal places and write the equation of variation.
c. Memphis (population: 650,000) is 400 miles from New Orleans (population: 490,000). Find the average number of daily phone calls, to the nearest whole number, between these cities.