unar is the free version and should be in Ubuntu

No. It’s ambiguous. In a math book or written by anyone that actually uses math, you don’t have a “%”

You group stuff below the line, and you use parens and brackets to group things like (a + b) and (x)(y) so that it is not ambiguous.

2/xy would be almost always interpreted differently than 2/x(x+y) which is ambiguous and could mean (2/x)(x+y) or 2/[(x)(x+y)]

No there is no clear right answer because it is ambiguous. You would never seen it written that way.

Does it mean A÷[(B)©] or A÷B*C