Its a statistical assumption - if people live for 80 years then 'statistically' every year 1/80th population has to die.

• If less than 1/80th population dies - then some people would end up living longer than 80 years
• If more than 1/80th population dies - then some people would live for much shorter

As mentioned in the previous answer - usually fair assumptions during the cases (to be validated with the interviewer) are:

1. population is evenly distributed so: 1/80 of pop is 1-yr old, another 1/80 is 2-yr old, etc., till 1/80 of the population is 80-yr old
2. population is not growing: the population remain constant at 320M 

given that if the average life is 80 years, statistically you can imagine that when people turn 80 they die (an no one die earler or later) > 1/80 of the population is dying every year (replaced by the same amount of new born to keep the population constant)

Hi there,

Very important for you to understand churn/replacement rate as market sizing!

If the average lifespan is 80 years, then every year you have a 1/80th chance of dying. If you have 80 people, one of those people will die every year.

Hence, dividing 320M by the % of people that die each year!