The number of phone calls between two cities during a given time period, N, varies directly as the populations p₁ and p₂ of the two cities and inversely to the distance, d, between them. If 100,000 calls are made between two cities 300 mi apart and the populations of the cities are 60,000 and 200,000, how many calls are made between two cities with populations of 125,000 and 175,000 that are 450 mi a part?

(a) Write the variation equation.

(b) Determine the quantity indicated.