This should be known as "gambler's ruin". In a game, at each step, you can win 1\$ or lose 1\$. Let $Z_i$ be a variable that can assume as values 1 or -1. Let $ X_n=\sum_{i=0}^n Z_i . $
Can you show me in details how to calculate $P(X_n \geq a)$ for a certain $a>0$? I thought that it was the case to use the cumulative binomial distribution, but I tried to compare my results with the data I have and they did not match.
As second question, I would appreciate just a little hint on how to compute that probability with excel.