Added to maths and strings (#1642)

* Added to maths and strings

* added changes suggest by cclauss

maths/combinations.py

@@ -0,0 +1,19 @@
+from math import factorial
+
+
+def combinations(n, k):
+    """
+    >>> combinations(10,5)
+    252
+    >>> combinations(6,3)
+    20
+    >>> combinations(20,5)
+    15504
+    """
+    return int(factorial(n) / ((factorial(k)) * (factorial(n - k))))
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()