I need help I haven't learned this yet so can someone please help

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

For a field trip to the zoo, 4 students rode in cars and the rest filled nine busess. how many students were in each bus if 472 students were on the trip

The total cost of anja’s trip to the dentist was $628.35. she paid a flat fee of $89.95 which included the checkup and cleaning and then had 4 cavities filled, each of which cost the same amount. which shows the correct equation and value of x, the cost of each cavity filling?