Edit typos and grammar and add python files

thinkcspy/Chapter-2-Exercises/Python-Files/problem7.py

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+# Runestone.Academy thinkcspy course
+# Chapter 2
+# Problem 7
+
+P = 10000 
+n = 12
+r = 0.08
+n = 12
+
+# Noticed that they gave separate variables to the inputs problem 3 and
+# the converted ints so decided to do the same here.
+t_string = input('For how many years will your principal be earning interest?')
+t = int(t_string)
+
+A = P*(1+r/n)**(n*t)
+
+print('The final amount after you will earn after', t, 'years is', A)