Yes, Mathcad will often give you some undesirable units in the result. in-lbf is a common one for me also; it is not the mathcad default unit. However, all you have to do it type over the units it gives you with the units you desire. (It will give you those typed units plus anything else it needs to make the units work, so if it's adding additional units you've formulated something wrong.) If that doesn't work please post an example. But I use lbf and lbm all the time with no issue (Prime anyway, I assume 15 is the same)
The only way you can control somewhat the behaviour of mathcad's working with units, is by telling it to not use any unit system. Then you can, no, have to, define your own unit system. The downside of this is that you have to tell mathcad with most every result what the unit is you want it to be displayed in. Basically that is the same as what you can do now: without defining your own unit system, but using a built-in one, just fill the units-placeholder of a result with the unit you want in there.
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