Fundamentals - Real Numbers - Chapter 1.1, Problem 1E

a. A natural number is a positive integer such as 1, 2, 3, 4, 5, etc. b. An integer is a whole number, or a number that is not a fraction such as 1 or -1. -1 is a correct answer because it is an integer but it is not a natural number, because it is negative. c. A rational number is a number that can be expressed as a fraction. $\frac{3}{7}$ is a correct answer because it is a fraction, and it is not a whole number so it is not an integer. d. An irrational number is a number that cannot be represented in a ratio of two integers. $\pi$ is an irrational number because it cannot be represented in a ratio of two integers.