Move Project Euler under puzzles.

puzzles/project_euler/p015_LatticePath.py

@@ -0,0 +1,19 @@
+#Lattice paths
+#Problem 15
+#
+#Starting in the top left corner of a 2x2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.
+#__   _    _       
+#  |   |_   |   |   |_    |__ 
+#  |     |  |_  |__   |_     |
+#
+#How many such routes are there through a 20x20 grid?
+#
+#Solution:
+
+a = 1
+b = 1
+for i in range(21,41):
+    a *= i
+    b *= i-20
+print( a//b )
+