Question 147075
In 1900 avg. life expectancy is 55. To make is 91, we need 91-55=36 more years

In 1 year it increases 0.2 years, So to increase 36 years we need
36/0.2 = 180 more years to make life expectancy 91.

So the birth year is 1900 + 180 = 2080 birth year.