Interpolate
Function 
Interpolate(x, x1, x2, y1, y2)

Interpolate is included by default in each file as part of the Math library.
See also:
Use
The Interpolate() function returns a y value given an x value and two sets of x/y pairs. It performs classic interpolation given all of the inputs as individual values.
If defined as a custom function in Blockpad, it would look like this:
Examples
Interpolate(13, 10, 20, 0, 10) = 3
Interpolate(4 ft, 0ft, 100ft, 0 lb, 10lb) = 0.4 lb