Question

Is o.123 rational or irrational

Answer 1

The definition of a rational number is:
Any number that can be made by building a fraction
using whole numbers.

You can make  0.123  that way.  Here's the fraction:   123/1000


It's also handy to know that any number that's completely
written down using digits ... and a fraction bar or a decimal
point if they're needed ... is a rational number.  This is not a
definition, and it doesn't cover all rational numbers, but it's
handy sometimes.

It's also handy to know that a decimal that has a set of digits
after the decimal point that repeats forever is a rational number,
even though you can't completely write it down (because it never
ends).