This just an example.

• Lets say L(y) is the function of getting life insurance. We could go further and say L(y) = 90-y which could be something like the percent chance of getting life insurance where x is your current age. We can see that past a certain age your chance of getting life insurance become zero.
• Next lets say A(x) is your age function, and again we could go further and define A(x) = 2013 - x, where x is the year you were born, and 2013 is the current year.
• So then the chance of getting life insurance could be expressed a L(A(x)) where x is the current year. So now we can expand.
• L(A(x))  The function L of A of x.
• L(A(x)) = 90 - A(x)              Substitute A(x) function in for y in the L(y)
• L(A(x)) = 90 - (2013-x)        Expand A(x)
• L(A(1995) = 90 - (2013 -1995)  Put in the year you were born for x
• L(A(1995) = 90 - 18
• L(A(1995)) = 78% chance of getting life insurance  at the age of 18.
• L(A(1960) = 90 - (2013 - 1960)  If you were born in 1960.
• L(A(1960) = 90 - 53
• L(A(1960) = 37% chance of getting life insurance at age 53.

I hope this helps,

Steven