You are doing nothing wrong and you should get the desired result.
The problem is well know and has to do with the units used. You are using pF in your calculations but Mathcads solve block will work with values in the default unit F. That way the values for C.shunt internally are so small, that we fall below a certain limit of numerical precision. Mathcads solve block usually does not distinguish between < and <=, so even if you demand C.shunt to be smaller than 120 pF, Mathcad would also be happy if its equal and the values are so small that for Mathcads precision they are considered equal. Setting CTOL to a smaller value should cure that problem, but unfortunately Mathcad solve block fails, not finding a solution at all.
You may have noticed that its also a matter of the guess value. If you set it to 11 pF you get the desired solution.
One way to cope with the problem is by multiplying the values in your inequation by a large number, like
A better way IMHO is to make the inequation unitless:
This solves the problem and you get the desired 101.9 pF without having to change the guess value.
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