URGENT please solve number 12 it’s due at 11:59 tonight thanks

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

If the cost to mail a package is an initial $5 and $0.28 for every pound the total cost to mail a package was $11.27, how much did the package weigh?

Solve this system of equations. 12x − 18y = 27 4x − 6y = 10