Chapter 5 - Number Theory and the Real Number System - Chapter Summary, Review, and Test - Review Exercises - Page 336: 91

$2 \div 3 = \frac{2}{3}$ ,which is not a natural number. Thus, the natural numbers are not closed with respect to division.

RECALL: The natural numbers are $1, 2, 3, ...$. Note that $2 \div 3 = \frac{2}{3}$, which is not a natural number. Thus, the natural numbers are not closed with respect to division.