Division by 0 is NOT ALLOWED in mathematics. Reason :-
Let's see the following scenario :-
Let the answer be x.
Thus 5/0=x , = 5=0×x, = 5=0.
See! What we proved! 5=0 ! Impossible.
Here the value of the answer (x) is also not defined because there is no such number which gives the answer 5 when multiplied by 0 (0×x=5; in this case x doesn't exist).
Thus, division by 0 is not allowed and undefined.
Hope you understand.