HIn 1991, The Life Expectancy Of Males In A Certain Country Was 72.8 Years. In 1998, It Was 75.2 Years. Let E Represent The Life Expectancy In Year T And Let R Represent The Number Of Years Since 1991?

In (1998 - 1991) = 7 years, the life expectancy changed by (75.2 - 72.8) = 2.4 years. The increase is (2.4 years/(7 years)) = 12/35 year/year.
  E(t) = (12/35)t + 72.8
  E(18) = (12/35)*18 + 72.8 ≈ 6.2 + 72.8 = 79.0
The life expectancy in 2009 is predicted to be 79.0 years.