Question 341947
 The number of long distance phone calls between two cities in a certain time period varies directly as the populations p1 and p2 of the cities, and inversely as the distance between them. 
---
n = k[(p1*p2)/[distance] 
If 10,000 calls are made between two cities 500 mi apart, having populations of 50,000 and 125,000, 

--------------
Solve for "k": the constant of proportionality

10,000 = k[50000*12000]/500
10,000= k[1200000]
k = 0.0083
-----
Equation:
n = 0.0083[P1*p2/distance]
---
find the number of calls between two cities 800 mi apart, having populations of 20,000 and 80,000.
---
n = 0.083[20000*80000/800]
n = 0.083[2000000]
n = 16666 2/3
---
Rounding down: n = 16,666 calls 
=================================
Cheers,
Stan H.
=============