Each assessment question on an assignment is worth the same amount of points (this includes Part A questions, Part B questions, and short-answer questions). For example, if a text has 4 assessment questions (say, a regular multiple-choice question, a Part A multiple-choice question, a Part B multiple-choice question, and a short-answer question), each question is worth 25 percentage points. If a text has 10 assessment questions, each question is worth 10 percentage points, and so on.
To find out the point value for a particular question, simply divide 100 by the number of assessment questions on a particular assignment. (Note: This does become a bit more complicated when a text has Part A/Part B questions. See this article for more info on that.)