Finished 15 a while ago, but for some reason I was missing the file.

atnan · Jan 30, 2009 · 94db8b3 · 94db8b3
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+# Problem 15:
+# Starting in the top left corner of a 2x2 grid, there are 6 routes (without 
+# backtracking) to the bottom right corner. How many routes are there through a 
+# 20x20 grid?
+
+require 'lib/factorial'
+
+grid_size = 20
+central_binomial_coefficient = (2 * grid_size).factorial / grid_size.factorial ** 2
+
+puts central_binomial_coefficient
+
+# Answer: 137846528820
+# Time: 0m0.010s