Trouble assigning variable to solver output

This is because the solution contains two ' variables'  that are unknown to the numeric processor: lbm and s.

You should tick the setting to allow units/constants in symbolics (under Calculation => calculation options), and see if that helps.

Success!
Luc

P.S. But the better way is to define mt as a function of its components and solve that:

And then you can calculate its numerical value with:

And do the same for the other two m-values.

I guess that setting the calc option to use units in symbolic calculations as explained by Luc will help (it not always does, though).

You may also consider to use the numeric (unit-aware) "root" function instead of the symbolic "solve" command. The "root" function can be used in two ways, either by providing a guess value or by providing an interval.