What’s the answer I’m confused

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

Two planes leave an airport at the same time, one flying east, the other flying west. the eastbound plane travels 20 mph slower. they are 1600 mi apart after 2 hr. find the speed of each plane.

The gdp is $1,465.2 billion. the population is 249,793,675. find the per capita gdp.