Rational numbers can be written as a fraction or as a/b. The "b" in a/b cannot be zero because you can't divide by zero. an example is 1.5. 1.5 can be written as 3/2/ Irrational numbers are numbers that can't be written as a fraction. A famous irrational number is pi. You can use this as a subject if you're a math teacher. Here is a video to help understand.