Jerryz the math are easy to explain what the statement means.
Xi are the ith step displacement. Assume step size equals S, so Xi can be either +S or –S.
Total displacement after n steps is
Xtot = X1 + X2 + X3 + …. + Xn
Mean [Xtot] = Mean[X1 + X2 + X3 + …. + Xn] =...