Project-Euler / 15
Problem 15 asks:
Starting in the top left corner of a 2×2 grid, there are 6 routes (without backtracking) to the bottom right corner.
How many routes are there through a 20×20 grid?
If you know anything about combinations in mathematics this problem is easy to solve without a line of code. You just use the binomial coefficient: $$\binom{40}{20} = \frac{40!}{20!^2}$$