Can you pls solve and give me the steps to solve

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

What is the first step of an indirect proof of a number that is divisible by two

Oscar and julie ashton are celebrating their 15th anniversary by having a reception hall. they have budgeted $4,500 for their reception. if the reception hall charges a $90 cleanup fee plus & 36 per person, find the greatest number of people that they may invite and still stay within their budget.