Define the function mean_val(dict_arg) that returns the average of all the values in dict_arg.

Here is a possible implementation of the mean_val() function in Python:

def mean_val(dict_arg):
    values = list(dict_arg.values())
    return sum(values) / len(values) if values else 0

The function takes a dictionary as input and extracts its values into a list. It then returns the average of the values using the sum() and len() functions. If the dictionary is empty, the function returns 0 to avoid a division by zero error.