Don't know why the units cause problems.
See if this helps https://community.ptc.com/t5/Mathcad/Using-Units-in-a-for-loop/m-p/84351
When you learn programming than one of the first things you learn is that you should never compare float values for equality but rather ask if the absolut value of their difference is smaller than a certain small value like 10^-10.
The use of the units results in very small floating point rounding/conversion errors and so (l-b) is not exactly equal to x.
You may do it that way:
Or define a function
BTW, your programs don't contain any loops (no "if" and no "while").
Because the value stored in x and the result of l-b are not exactly equal. Because of internal round off or conversion errors they are off by about 10^-18 as I had shown. Thats the reason why if x=l-b ... does not trigger - Prime realizes/thinks the the expressions are NOT equal.
If you have two real floating point values (and using unit mm seems to turn the integers into floats) a and b you should not ask if a=b but rather if "a" is approximately equal to "b" within a certain accuracy. So you don't ask "if a=b ..." but rather "if |a-b|< eps ..." with eps being a small value like 10^-12 or the like.
But there is an even simpler solution to your problem: Chose menu ribbon "Calculation", then open "Calculation Options" and turn on "Approximate Equality". This should make your if statement trigger the way it should be.
