Question 169536
<pre><font size = 4 color = "indigo"><b>
First let y represent the life expectancy and let x 
represent the number of years since 1990.  Then later 
we'll replace y by E(t) and x by t.

Then when x=0, then y=72.4, and
     when x=4, then y=74.9

So the problem now becomes:

Find the equation of the line which contains the points
(0,72.4) and (4,74.9).

Use the slope formula:

{{{matrix(1,9,
     m, "=", (y[2]-y[1])/(x[2]-x[1]), "=", (74.9-72.4)/(4-0), "=", 2.5/4, "=", .625) }}}  

Use the point-slope form:

{{{y-y[1]=m(x-x[1])}}}
{{{y-72.4=.625(x-0)}}}
{{{y-72.4=.625x}}}
{{{y=.625x+72.4}}}

Now replace {{{y}}} by {{{E(t)}}} and {{{x}}} by {{{t}}}

{{{E(t)=.625t+72.4}}}

To find the life expectancy after 18 years from 1990.

{{{E(18)=.625(18)+72.4=83.25}}}

Since 18 years from 1990 is this year 2008, then if that
formula is a correct predictor, then the life expectancy
at present should be {{{83&1/4}}} years.

Maybe it'll be over 100 years by the time you get old!

Edwin</pre>