Solve each variation problem:

1. If m varies jointly as z and p, and m=10 when z=2 and p=7.5, find m when z=5 and p=7.

2. Current in a circuit: The current in a simple electrical circuit varies inversely as the resistance. If the current is 50 amps when the resistance is 10 ohms, find the current if the resistance is 5 ohms.

3. Long Distance Phone Calls: The number of long-distance phone calls between two cities in a certain time period varies directly as the population P1 and P2 of the cities, and inversely as the distance between them. If 10,000 calls are made between two cities 500 miles apart, having populations of 50,000 and 125,000, find the number of calls between two cities 800 miles apart, having populations of 20,000 and 80,000.

4. What happens to y if y is inversely proportional to x, and x is tripled?