Robert A. Beezer, David Farmer, Alex Jordan, Mitchel T. Keller
An <exercise> can be rather freeform, containing elements such as <p>, <figure>, <image>, etc. However, an author will typically think about an exercise as having an associated correct answer, perhaps with a hint or a detailed solution. PreTeXt has tags to support that sort of content, but it does require that your <exercise> be structured. This is nearly identical to the structure of a project-like element.
Note that you can have multiple <hint>, <answer>, and <solution> elements. But you must put all the <hint>s first, then all the <answer>s, and then all the <solution>s. There are a variety of options for determining where hints, answers, and solutions appear (at all). Check Section 5.5 for information about stringparams.
An <exercise> can also have a more complicated structure that assigns a sequence of steps for a student to complete. PreTeXt provides the same <task> tag that is used in project-like elements to give structure to such an <exercise>.