Traceback (most recent call last):
  File "ex4.py", line 8, in 
    average_passengers_per_car = car_pool_capacity / passenger
NameError: name 'car_pool_capacity' is not defined

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explanation --> probably because this line don't exits:
                carpool_capacity = cars_driven * space_in_a_car
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Here's more extra credit:

* I used 4.0 for space_in_a_car, but is that necessary? What happens if it's just 4?
Nothing, the result is the same.

* Remember that 4.0 is a "floating point" number. Find out what that means.
The result, using floating point numbers will be a floating point number.