The formula below from Blitzer (2018) gives a fairly accurate value for the number of daily phone calls, C, between two cities whose populations are P1 and P2 where d2 represents the square of the distance between them. This is an example of simultaneous joint and inverse variation or combined variation with the proportionality constant, k, equal to 01778 c equals .01778 times P1 times P2 divided by d squared use the formula above to calculate the distance in miles between Dallas, Texas and Austin,Texas.