All rational numbers can be expressed as a/b where a and b are integers and b is non-zero, if the numbers cannot be expressed in this way then they are irrational. For example 5.25 = 21/4. Here 5.25 can be expressed as a ratio of two integers, so 5.25 is a rational number. In another example √27 = 3√3 but it cannot be expressed as a ratio of two integers, therefore √27 is an irrational number.