This is a real strange effect. I know that the conversion from decimal to binary and back can result in numeric inaccuracy. Especially as numbers like 0.7 or 0.1 have a periodic binary representation which has to be cutted somewhere. But usually there should be enough reserves to cope with that effect in case of a simple multiplication of two numbers. I was not able to reproduce that effect with other numbers than 0.7*11. So I hesitate to call it a bug.
The effect shows in Prime, too, but only in the symbolic eval. In Prime we cannot set the number of decimals to anything higher than 15 (17 in Mathcad 15), but in MC15 the effect shows already with 15 decimals whereas in Prime even with 15 decimals set I see the exact 7.7. Nevertheless the symbolic eval shows in Prime too, that the value in x is not exact.
Another effect which is somewhat beyond my understanding is, why the symbolic eval does not show the inaccuracy if a unit is added!!??
When in MC a calculation is stored to a variable, there are two things stored: the numeric value, which is used for subsequent numeric calculations and the "history", how the result should be derived (I guess that thats not really stored to the variable but that for symbolics the original assignment is referenced instead of the stored numeric value). Thats the reason why z:=0.7*11 folowed by z --> would show the correct symbolically evaluated 7.7.
If the assignment is followed by a numeric inline evalulation, only the rightmost expression is stored and thats the (inaccurate) numeric eval. Thats the reason why x is shown inexact even when evaluated symbolically - the "history" (=multiplication 0f the two numbers) is not stored anymore.
This seems to be different when a unit (an unknown variable to the Symbolic) is added. I would have guessed that, because of the inline eval, 7.699999999993 * m would be referenced for the symbolic, but it looks like this time the real history (=the two factors 0.7 m and 11) is taken - why?
Even writing y:=(0.7*11)*m doesn*t change anything.
But then we have
So the symbolics take the unit from the rightmost eval but the factors from the first assignment?????