Help fast plz are they similar or not

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

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Determine the fraction of total interest owed. after the fourth month of a 12-month loan: the numerator is: {(n + ) + (n + ) + (n + ) + (n + )} = , and the denominator is: {(n) + (n + 1) + + (n + )} = . therefore, the fraction is numerator/denominator (to the nearest tenth) =