Question 20565
First of all,let us start be defining the term "closed"
A set of numbers is said to be closed for an operation, if upon performing that operation between the numbers you get a member of that set.
For e.g
Consider the set of whole numbers. Whole numbers are (1,2,3,4,5......)
Now,take any 2 numbers and add them. Say 5 and 6. The sum we get is 11 which as we know is a whole number.
Now we can say that the set of Whole Numbers is closed under Addition.
Consider Subtraction.
6-5 = 1
But 5-6 = -1 which is not a whole number.
So  the set of Whole Numbers is NOT closed under Subtraction.

Can you try for Multiplication and Division?