Which set of integers is not closed under what operation?

Question

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Division
I thought it was division but I don't understand , can you please explain this.
Thanks

Answer 1

Think of it this way:
If you add two integers, will you always get back an integer ? YES , so it is closed under addition

if you divide two integers will you always get back an integer.
e.g. 6 ÷ 4 = 1.5 , which is NOT an integer
so not closed under division

make a similar test for subtraction and multiplication.
What do you think?

Answer 2

6*4=24 it is an integer=closed for mult.
10-7=3 so it is also an integer
Thanks