有r个人在一楼进入电梯，楼上共有n层。设每个游客在任何一层楼下的概率相同，试建立一个概率模型，求直到电梯中的人下完为止，电梯需停次数的数学期望，对r=10，n=8（如西一楼）进行计算机模拟验证。(There are r people in the first floor into the elevator, upstairs, there are n layer. The probability of each visitors at any level below the same, try to establish a probabilistic model for the people under the elevator until the end date, mathematical expectation, lift to stop times of r=10, n=8 (such as the West Building) computer simulation.)