有一个神奇的口袋，总的容积是40，用这个口袋可以变出一些物品，这些物品的总体积必须是40。John现在有n个想要得到的物品，每个物品的体积分别是a1，a2……an。John可以从这些物品中选择一些，如果选出的物体的总体积是40，那么利用这个神奇的口袋，John就可以得到这些物品。现在的问题是，John有多少种不同的选择物品的方式。 -Have a magic pocket, the total volume is 40, this pocket can conjure up some of the items, the total volume of these items must be 40. John now want to get the n-th items, the volume of each article are a1, a2, ... an. John can be selected from these items, and if the total volume of the selected object is 40, then the use of this magical pocket, John can obtain these items. The question now is, John has a number of different choice items.