The cost of getting a new life insurance policy depends on the age of the applicant. This can be expressed as C = F(t) where C is the cost; it is a function of t which is the age. F(t) is usually a very complex function using which insurance companies accommodate the risk they are taking on and how the probability of a person dying is dependent on age.

The age of a person is dependent on the year of birth. This can be expressed as t = F'(b) where t is the age and b is the year the person was born in. F'(b) is a simple function and equivalent to the difference between the present year and the year in which the person was born.

Using these functions the cost of the life insurance can be expressed as C = F(F'(b))