Interpolation is a mathematical technique which was popular before we had a lot of cheap computing power. The basic idea is that if you’re given a set of data and looking for a value in the same range, you can interpolate it to get a reasonable estimation for the value that is not actually in the set.
If you can find an old calculus, finance, statistics or algebra book, they had lookup tables in the back. Remember that the only computational tools students had back then were pencil and paper or a slide ruler. If you wanted to use a