you won't fail. unless they are giving you a very high tech test. there are basically two types of test.

1. the type of test parents give to their teenage kids at home. these tests are pretty cheap and found at the drug store. they can detect THC within the last id say, 5-7 days. if you drink a lot of water and make sure you pee a lot before this type of test you should be fine.

2. then there are the type of tests that real certified doctors due. these tests are pretty crazy and measure a number of different levels of things in your pee. you can't really get around these unless you have a kit (some doctors will sit and watch you take your piss) if you get one of these tests you might be screwed they can detect THC anywhere from 27-31 days.