I have a math problem. It is 18 : 15.4. Would i do 18 : 15 or 18 : 5?

Your question isn't stated in a very clear way so I'm going to show a few possible solutions for the various questions you may be asking.

1. 18 : 15.4

If you can use a calculator just plug it straight in. No need for the other things that you stated.

If not, once again do not do 18 / 15 or 18 / 5. Just do long division.

2. 18 : 15 (and the 4 is just a typo)

In this case, you should do (18 / 3) : (15 / 3). This is because the number 3 is common between both 18 and 15. The answer in this instance would be 6/5, or just Six-fifths