Imagine it like this.
The numerator and denominator of the fraction...
3abc
-----------
abc
are playing a dating game and their perfect match is someone exactly like them.
numerator A is a match for denominator A
and B for B
C for C ...
- But what is left all alone with no match ?
3 is.
3 is your answer -
A/A , B/B , C/C eliminate each others .
if you plug in numbers for A B C it would still work - Example :
a = 3 b= 2 c= 1
3/3 .... a/a
2/2... b/b
1/1.... c/c