Water company billing system

The water company is implementing a new billing system. For each house, the invoice is made as follows:

1. The first 50 litres are free.

2. Between 50 and 200 litres, each litre is charged at 0.10 euros.

3. From 200 litres on, each litre is charged at 0.30 euros.

4. The minimum fee is 6 euros, that is, if the amount to pay is less than 6 euros, then the payment will be 6 euros.

Write a program that calculates the water consumption of a family in a month given the number of litres used.

>>> %Run 
    Enter the litres of water used: 0
    The expense is 6.00€
>>> %Run 
    Enter the litres of water used: -500
    Please enter a correct value
>>> %Run 
    Enter the litres of water used: 300
    The expense is 90.00€
>>> %Run 
    Enter the litres of water used: 55
    The expense is 6.00€

Insist that the students test their programs by giving them example
test executions.