Brilli the ant decides to have a walk along a 1D line. For each step he takes, he would either walk forward or backward by a distance of 1 with equal probability. What is the expected number of steps Brilli would take before he reaches a distance of 2016 away from where he started?
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Let E n be the expected number of steps that Brilli must take to reach a distance N from his starting point, given that he is currently a distance of n from his starting point. Then we have the equations E n = 1 + 2 1 ( E n + 1 + E n − 1 ) 1 ≤ n ≤ N − 1 together with the boundary conditions E 0 = 1 + E 1 E N = 0 Solving these equations, we obtain E n = N 2 − n 2 0 ≤ n ≤ N and hence E 0 = N 2 . With N = 2 0 1 6 , this makes the answer 2 0 1 6 2 = 4 0 6 4 2 5 6 .