def scale(factor, lst):
    '''
    Purpose: Scale the elements of lst by a constant factor.
    Pre:  factor is a number.  lst is a list of numbers.
    Post: Mutates lst by multiplying each element by factor.
          Returns nothing.
    '''

    # We need the index, not just the value, so we can assign
    # to lst[i].
    for i in range(len(lst)):
        lst[i] *= factor

    # Return nothing (could just omit this line)
    return

def main():
    numbers = [ 11, 16, 20, 3, 99 ]

    print("Before scaling:")
    for n in numbers:
        print(n, end=" ")
    print() # end the line

    factor = float(input("Enter a scaling factor: "))

    scale(factor, numbers)
    # That call modified the 'numbers' list!

    print("After scaling:")
    for n in numbers:
        print(n, end=" ")
    print() # end the line

main()