HELP ME PLSs

Determine whether the set of whole numbers is closed under addition. explain why the set is or is not closed. give an example.

Aproduction manager test 10 toothbrushes and finds that their mean lifetime is 450 hours. she then design sales package of this type of toothbrush. it states that the customers can expect the toothbrush to last approximately 460 hours. this is an example of what phase of inferential statistics

If value of cos330° is √3/2 then find tan165°.