Solved 72 in Python.

python/e071.py

@@ -29,6 +29,22 @@ def euler_071():
     return n_min
 
 
+def euler_071_farey():
+    """
+    https://www.cut-the-knot.org/blue/Farey.shtml
+    n_min must be located between 2/5 and 3/7 for F_7
+    To calculate F_d where d <= 10**6 we continously calculate the
+    median between 3/7 and the new fraction.
+    """
+    n, d = (3, 7)
+    n_D, d_D = (2, 5)
+    while d_D + d <= 10**6:
+        n_D = n_D + n
+        d_D = d_D + d
+    return n_D
+
+
 if __name__ == "__main__":
+    print(euler_071_farey())
     print("e071.py: " + str(euler_071()))
     assert(euler_071() == 428570)