Describe how to use mental math to determine if a ratio is simplified

We can determine that the ratio is simplified by checking that the numbers in ratio can be divided with the same number, if it can be divided then it is not simplified for example 9:12 is the ratio and we can divide both numbers with 3 and then we get 3 : 4.

Now we cannot divide 3:4 further with any same number so this is simplified form.