I guess you're learning how to Python. The other answers are right. But I am going to answer your main question: "how to calculate percentage in python"
Although it works the way you did it, it doesn´t look very pythonic. Also, what happens if you need to add a new subject? You'll have to add another variable, use another input, etc. I guess you want the average of all marks, so you will also have to modify the count of the subjects everytime you add a new one! Seems a mess...
I´ll throw a piece of code where the only thing you'll have to do is to add the name of the new subject in a list. If you try to understand this simple piece of code, your Python coding skills will experiment a little bump.
#!/usr/local/bin/python2.7
marks = {} #a dictionary, it's a list of (key : value) pairs (eg. "Maths" : 34)
subjects = ["Tamil","English","Maths","Science","Social"] # this is a list
#here we populate the dictionary with the marks for every subject
for subject in subjects:
marks[subject] = input("Enter the " + subject + " marks: ")
#and finally the calculation of the total and the average
total = sum(marks.itervalues())
average = float(total) / len(marks)
print ("The total is " + str(total) + " and the average is " + str(average))
Here you can test the code and experiment with it.