购买时价格= (10%*100)/(1+10%)+(10%*100)/(1+10%)^2+(10%*100)/(1+10%)^3+(10%*100)/(1+10%)^4+(10%*100+100)/(1+10%)^5
那第三四五年利率是什么?10%还是持续20%呢?
鲁滨逊24*2=48和星期五24*4=96，机会成本,1:1 , 3:2
