用时间步长法实现：某仓库有一个保管员，领料工人的到达为Poisson流，平均每小时有12个工人来领料，每个工人的领料时间均匀分布在2-4分钟之间，求该保管员忙的概率。-Time step method: a custodian of a warehouse, picking the arrival of workers to a Poisson stream, hour average of 12 workers picking, each worker picking time uniformly distributed between 2-4 minutes , find the probability that the custodian busy.