Plumbing Code and Regulations
You cannot plumb legally from memory of a model code; you must know which document your authority having jurisdiction (AHJ) has adopted and in what edition. The three model plumbing codes in U.S. use are the International Plumbing Code (IPC), the Uniform Plumbing Code (UPC), and the National Standard Plumbing Code (NSPC). Each is amended locally, so a dimension that is correct in one city can be wrong thirty miles away. Questions built on 'the code' usually assume the model text, but a question that names a jurisdiction is testing whether you read the adoption clause.
Know the hierarchy of authority: the adopted code and local amendments first, then the standards the code references (ASME, ASTM, ASSE, NSF), then the manufacturer's listed installation instructions, which are enforceable because the listing is what makes the product legal. When two requirements conflict, the more restrictive governs; when a manufacturer is stricter than the code, follow the manufacturer.
The administrative half of the code is heavily tested: when a permit is required, who may pull it, what must appear on the drawings, the sequence of rough-in, top-out, and final inspections, and the water and gas tests performed before concealment. Definitions matter, and questions love the difference between a mandatory 'shall,' an advisory 'should,' a recommended practice, and a manufacturer's recommendation.
Worked example: a clearance is given as 1/4 in per foot of run, and you must state the drop over 12 ft in millimeters. Drop = 0.25 x 12 = 3 in; 3 x 25.4 = 76 mm. Convert first, round last, and confirm whether the answer options are inches or millimeters.