Defaults you can rely on
Most states share these defaults: 25 mph in residential and business districts, 15 mph in school zones during posted hours and in alleys, 55 mph on undivided rural roads, and 65 to 80 mph on rural interstates depending on the state. Highest in the country is 85 mph on a small section of Texas. Lowest practical city limit is 15 mph in school zones.
Basic speed law
Posted limits are the maximum legal speed in ideal conditions. In rain, snow, fog, traffic, or near a school release, the safe and legal speed is lower. The 'basic speed law' in most states gives officers authority to ticket you for driving the posted limit when conditions don't allow it. The test answer is almost always the conservative one: slow down for conditions.
Why minimums exist
Highways have minimum speeds (often 40 or 45 mph) because slow vehicles are nearly as dangerous as fast ones. If you cannot maintain the minimum, take a different route. Big speed differences between cars in adjacent lanes are a leading cause of multi-car crashes.
School zones
School zones use posted hours, flashing lights, or 'when children are present' language. When the lights are flashing or the times apply, the limit drops sharply, often to 15 or 20 mph. Fines in school zones are typically doubled. The test is checking that you know the limit applies even when you don't see children, as long as the time window is in effect.