Problem:

A town uses a two-tier system to calculate bill for water usage.  For the first 8000 gallons, it is $0.004 per gallon.  After the first 8000 gallons, the rate will be $0.007 per gallon. Suppose a customer uses 8500 gallons of water.  He will pay $0.004 per gallon for the first 8000 gallons, and $0.007 per gallon for the last 500 gallons. Write a Python program to determine how much a customer needs to pay.  Ask the user how much water he has used.  Calculate and display the water bill. The following are two examples.

Enter gallons of water used: 2100

Please pay this amount: $8.4

Enter gallons of water used: 8500

Please pay this amount: $35.5