4, if you multiply them.
5
There are 15 of them.
2x2=4
Yes. The entire set of natural numbers is closed under addition (but not subtraction). So are the even numbers (but not the odd numbers), the multiples of 3, of 4, etc.
There are infinitely many possible answers: Addition: 1,1,1,1,196 1,1,1,2,195 -1,-1,-1,-3,206 and so on. Or multiplication: 1,1,1,1,200 0.1,0.1,0.1,0.1,2000000 and so on. It is also possible to have combinations of addition and multiplication: eg 4*5 + 4*5*9 And then there are functions other than addition and multiplication.
4, if you multiply them.
There are infinitely many answers and these depend on what is done with the two numbers: addition, multiplication, exponentiation and so forth.In each case, there are infinitely many answers. For example, with addition,67.2 + 67.267.1 + 67.367.0 + 67.4...0.1 + 134.30.0 + 134.4-0.1 + 134.5-0.2 + 134.6etc.Then you can add numbers to two decimal places, to 3 do, 4 dp, ... infinitely many dp.
4
5
Yes - due to commutability, you can rearrange the numbers in any addition sum and the answer will still be the same.
There are 15 of them.
1+2+3+53 is one quartet. There are an infinite number of possible quartets for addition alone. And then you can have sutractions. And then multiplications, and divisions.
This is not possible, 27 is an odd number, 4 odd numbers will always equal an even number.
143
20
2x2=4