Question 1163713
.
<pre>

It is a standard linear interpolation-extrapolation formula


    y = {{{y[1]}}} + {{{((y[2] - y[1])/(x[2]-x[1]))*(x-x[1])}}}.


With  {{{x[1]}}} = 100,  {{{y[1]}}} = 16,  {{{x[2]}}} = 75,  {{{y[2]}}} = 13,  x = 800,  this prediction formula becomes


    y = 16 + {{{((13-16)/(75-100))*(800-100)}}} = 16 + 0.12*700 = 100.00  dollars.



When the question is posed as in this post, the person "should not think".


He (or she) must know the formula (or use a textbook or handbook or his memory) and follow it literally.


The only thing the user must understand, is who are the "Xs" and who are the "Ys".



It is also useful to understand that the formula express a straight line, passing through 

the points  ({{{x[1]}}},{{{y[1]}}})  and  ({{{x[2]}}},{{{y[2]}}})  in a coordinate plane.
</pre>

If you don't know this formula, then memorize it (!)