Thank you, Coryphaeus and Kees. I am thinking of "in general", without the use of the character map. I have been trying to find out how:
This is what I have found:
Superscript: highlight the text and press combo Ctrl+Shift and + .
Subscript: highlight the text and press combo Ctrl plus + .
It follows that to type any fraction character a/b, one just have to turn the numerator to a superscript and the denominator to a subscript.
Perhaps there are "correct" methods ?
Unfortunately, the process cannot be demonstrated here.
During my experimentation, I found that typing Alt and + combo ( at MS Word) turns the cursor to a big bolded "+", and Alt plus - combo turns the cursor to a big bolded "-". Wonder what these are for. Thanks.