This is not only a lesson in the math behind interpolation but a great lesson in recalling the formula to interpolate. Interpolation is a simple concept, the steam data we have in use today is determined experimentally. Thus there is individual experiments for each of the data point, and we will therefore be missing some data point you are looking for.
Interpolation gives us a method to determine the point you are after by simply drawing a straight line between the two points. This method is simply termed linear interpolation and is explained in the video.
Unfortunately we must memorize this technique and the formulas associated with it. Thus I hope the following makes it quite easy for you:
As always please comment below for any clarifications or improvements and send any problem requests to my email.