Question

Which of these numbers is irrational?
A. 12
B. -4.2
C. 1/42
D. 4.2

Answer 1

A: 12

Explanation:

An Irrational Number is a real number that cannot be written as a simple fraction. (Irrational means not rational).

Therefore, we can immediately discount C as 1/42 is a rational number by inspection.

`4.2=(4.2)/(1) =(4.2)/(1) * (10)/(10) =(42)/(10)=(21)/(5)` so similarly -4.2 = -21/5

So as B and D can also be written as rationals, they are not irrational.

Therefore, A is irrational.