some possibilities

acer — Sun, 05 Dec 2010 08:25:18 Z

The problem is that the units in numerator and denominator do not simplify and cancel by default.

> a/(b+t*Unit(kg));

                      7 Unit('kg')            
             -----------------------------
             10 Unit('kg') + t Unit('kg')

You can make them "cancel" in several ways.

> a:=7*Unit(kg):
> b:=10*Unit(kg):

> y:=t->ln(a/(b+t)):
> plot(combine(y(t*Unit(kg)),units));

> y:=proc(t) uses Units:-Standard;
>            ln(a/(b+t)); end proc:
> plot(y(t*Unit(kg)));

> y:=t->ln(a/(b+t)):
> plot(simplify(y(t*Unit(kg))));

acer

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some possibilities