Undefined???? 4÷0+20?

Because you can't divide by zero. This is undefine, or really makes no sense. Imagine if I said I drove 20 miles with no vehicle.

Division on zero is not defined.

you cannot divide by zero becaue the result would be INFINITE, so the only way you could state this expression is INFINITY PLUS 20.

4/0 + 20
= undefined

May be written as :-
(4 ÷ 0) + 20 which is UNDEFINED because 4 ÷ 0 is UNDEFINED.

For the above expression you would surely have to divide by zero according to the rules,that gives us 4/0 which is undefined or infinity.Now when twenty is added to it,it still remains infinity.As you see if you have infinite books n u gave some to your friend you would still have infinite number of books left with you.Or if your friend gave you some books and you added those with the infinite books you have,they would still remain infinite.This is because infinite means not finite and which cannot be counted,so it doesnt matter if u add or subtract anything from it,it would still remain infinite.

Take care:)

depends on what rules you use,
some people think that you can't divide a number by zero, so then it would be undefined.

in early years, I thought that the zero meant, "it hasn't been divided by anything", which DOES work