Question

radio gives local weather updates every 18 minutes. the first update is at 2:00pm. you turn the radio on at 5:30pm. how long will you have to wait for the next local weather update?

Answer 1

youll have to wait 10 mins

Answer 2

So this is just my guess. Sorry if It turns out incorrect. 
If at 2:00pm you hear the first weather update, and every 18 minutes you get a new update then you just keep adding 18minutes to your time. That's what I did.  Hopefully this makes sence to you.  
2:00+18= 2:18pm you have 2 updates, 2:36pm you have 3 updates, and evnetually at 5:20pm you will have a total 12 updates. The girft here is strange though because it is asking "How long will you have to wait for the next local uodate?" Well add 18 to 5:20pm. That equals 5:38pm. But, you turn the radio on at 5:30pm. That means you will have to wait 8 more minutes until the next update comes on.