1. no idea
2,need estimate an original value before GIVEN
i think, no units in equation set is better
1. Why does the "Answer" not simplify the units?
Because you are evaluating symbolically and the symbol processor in Mathcad knows nothing about units. Units are treated as unknown variables and the symbol processor does not know that Ampere times Ohm is Volt. Normally you could follow the symbolic eval immedeatly by a numeric eval (=), but in your case this is not possible, because the solution vector consists of values with units of different dimensions. Mathcad 15 and below unfortunately does not allow for this. So we have to assign the values to single variables to be able to use them in further calculations (see below).
2. Why can't I ever get the red "f" to go away?
This error is thrown by the numeric processor in Mathcad because it would need guess values for the searched for variables. But as you have seen, the function can be evaluated symbolically nonetheless. You can get rid of that error by either providing guess values (as shown by Pirates above) or by evaluating the Find() command symbolically as shown here.
BTW, I guess you know that you can get rid of R.2 as it is never used and that you could simplfy your system to just two unknown as I.1 is known to be equal to I.S.
