Today, my case partner asked me to estimate the number of weddings per year in a given country. Let's arrive to the core of the case in simplified terms. After dividing the population in four age sections, we came up with this situation:

0-20: 0 weddings

21-40: 200 weddings

41-60: 100 weddings

61-80: 20 weddings

Now, in this given population we have 320 potential weddings. I was going to divide 320 weddings by 60 years, but my case partner stopped me and told me that I had to divide by 20 years (200/20 + 100/20 + 20/20 or 320/20).

The two approach are not equivalent, hence one is wrong. Which one and why?

PS: If dividing by 20 years is correct, I have another doubt. Let's say that we approach the case differently and we segment the population in only one big age cluster (21-80). Now, in order to have the same number of weddings per year (320/20 = 16), the number of potential wedding in the same population should be 16*60 = 960. For this reason, I suspect that dividing by 20 is not the correct option.

Many thanks for the effort!

(edited)