Each cereal box has one of n toys, equally likely. How many boxes, on average, until you have all n?
每盒麦片随机送 n 种玩具之一。平均要买多少盒,才能集齐 n 种?
设已经集齐 k 种,还要买的盒数为 Tk,每盒是新种类的概率是 (n − k)/n,所以它服从几何分布。
这样讲:“Each new toy is a geometric wait, so I add up n over n minus k, which is n times the harmonic number.”



