I need help finding the average number of words per sentence, within a list

[elbert.leach]

# This program reads numbers from a file into a list.
def main():
    # Open a file for reading.
    infile = open('text.txt', 'r')
    sentences = infile.readlines() # Read the contents of the file into a list.
    infile.close()                          # Close the file.


    index = 0                               # Convert each element to an int.
    while index < len(sentences):



        print (sentences[index], index) #PRINT EACH ELEMENT
        index += 1

        # Print the contents of the list.

    print (index)


# Call the main function.
main()

[Gribouillis]

Take a page containing a list of sentences, like this one (take only the first 5 sentences). Then with a calculator, compute the average number of words per sentence by hand and note carefully everything you do. This should give you a working algorithm. Write pseudo code, then python code.

[Ene Uran]

Assume the sentences end with '.' or '?' or '!' so count these characters and divide the total words by the number of these characters.