Ratio 18:24 in lowest terms

Treat it as a fraction. Divide by 6, gives you 3:4.

3 is a prime number, if you bump into a prime you won't be able to divide any further.

First you would find the highest number that can evenly divide into both 18 and 24, that number happens to be 6, so 18 divided by 6 is 3 and 24 divided by 6 is 4, so 18:24 in lowest terms is 3:4.