I just had fluoride treatment, & ate after 30mins. can i brush my teeth after eating? if not, how long should i wait until i can brush my teeth?thanks

Should be fine: You didn't really specify what form of Fluoride treatment you had. If it was a gel or foam then most of the Fluoride was washed away with saliva and food when you ate. If it was a varnish of 5% strength you probably should have avoided eating or drinking for about an hour. Either way you should be fine to brush right away.

Lag time: Unless your Dentist/Hygienist told you not to eat for a specific period of time, you're good to go.