Chapter 5 - Number Theory and the Real Number System - 5.5 Real Numbers and Their Properties; Clock Addition - Exercise Set 5.5 - Page 310: 91

The statement makes sense. Create a subset of integers: $\left\{1, 2, 3\right\}$ $1+3=4$, which is not an element of the set.

The statement makes sense. To illustrate, if the subset $\left\{1, 2, 3\right\}$ is taken, $1+3=4$, which is not an element of the set. Thus, this subset of integers is not closed under addition.