Ladies paper steve

How well do the teeth whitening trays at the dentist work?!?!?!? - tooth whitening trays

I'll get my teeth and my dentist advised me to lighten it. Who knows how many shades (on average) to whiten your teeth? I do not understand the nature of drawers, which are cheap, the business .. I'm @ the dentist. Thanks in advance!
PS I need to know really fast,