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
