小弟有一道题怎么也想不出，麻烦各位前辈帮忙解答一下~
题目是这样的：
Assume that as at 1 January the male population at age 20 last birthday numbered 135,738 and that the hazard rate is constant between exact ages 20 and 21 and equal to 0.0019. Determine the probability that there will be more than 157 deaths during the year.

下面是答案的截图：


我的疑问是，为什么135,738在这里要作为total waiting time (or central exposed-to-risk), i.e. E[V]来用，而不是作为N number of i.i.d. individuals来用？