All depends on your metabolism. At 20 days it is possible to pass, due to having almost 3 weeks, and from your description you are thin, and that helps. Buy some test strips and start doing a first piss of the day test. When you can pass that your almost there. Keep reading knowledge IS power. If you cannot pass that a day or 2 prior then substitution is your way to go. It should be relatively easy, after a few practice runs. Don't wait to start practicing the sub, it will make you much more at ease. You will need a digital thermometer and condoms that have been rinsed out. Check your temps and get a good idea wheather you can depend on body temp to hold you in range and for how long. Scope out the test facility, and make note of anything that you can that will be of help. Being female they will give you more privacy than males. Preemployments are not generally observed unless they detect that you are doing something to subvert. Make sure that it is urinanalisis, and not hair test. There is a completly different methodology to pass if it is hair testing. Post back with your progress, something you discover may help others. Please help yourself by reading both the dillution and substitution stickys. God I wish this crap was illegal.
STIMPY21