Somehow your sheets seems to got corrupted. When I open it with the current version of Mathcad 15 (M030) I get a very unusual error:
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When I click OK r and q are variables known to the numeric processor but not to the symbolics.
I retyped your expressions in a fresh worksheet and all worked as expected:
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Note that I avoided the inline evaluation after the explicit eval. The reason is the way Mathcad stores numbers and units and the way the symbolic processor sees it.
The symbolic processor is not aware of units and treats them as unknown variables. So it cannot convert in to ft, e.g.
if you define a:=2in b:=3in c:=a+b (explicit or not does not matter here) c is stored internally with unit in and if you evaluate c symbolically you will get c-->5in.
But if you define c:=a+b...=, the numeric eval(=) will convert the unit to the standard length unit (ft in your sheet, m in mine) and if you symbolically evaluate c you will see the result in ft (or m) as thats the only "unit" ze symbol processor will see. It does not help to manually set the unit to a different one after the numeric eval.
