Directions: answer true or false. if false, provide a counterexample.
16. natural numbers are closed under division.
17. negative numbers are closed under addition.
18. prime numbers are closed under subtraction.

Natural numbers are closed under division: false.

A set is closed under a certain operation if the results of that operation are always inside that set.

So, if natural numbers were closed under division, the division of two natural numbers would always be a natural number.

You have plenty of counterexamples, to pick one you may divide any odd number by 2: 5/2 is not a natural number.

Negative numbers are closed under addition: true.

Let be two positive numbers. So, are two negative numbers. Their sum is

And since is positive, we deduce that is negative, so the sum of two negative numbers is still negative.

Prime numbers are closed under subtraction: false.

This would mean that the subtraction of two primes is also a prime. Again, there are many counterexamples: 7 is prime and so is 3, but their difference 7-3 is 4, which is not prime.