No.
A set is closed under multiplication if for any two elements, x and y, in the set, their product, x*y, is also a member of the set.
Yes.
yes
Yes
Is { 0, 20 } closed under multiplication
No.
No.
No.
No.
Yes they are closed under multiplication, addition, and subtraction.
Rational numbers are closed under addition, subtraction, multiplication. They are not closed under division, since you can't divide by zero. However, rational numbers excluding the zero are closed under division.
Yes!
Yes!
Yes.
Yes.
Because the product of any two elements is also an element of the set.