You are not only conveying information, but also proving to your audience--the examiner--that you have mastered the information and can work with it.
In other words, your purpose is both informative and persuasive.
You want to show your instructor that you have mastered the material. Although you will be working under pressure, take a few minutes to plan your time.
Determine how many minutes you can devote to each answer.
In this context at least, quality is always preferable to quantity.
Also check sentence structure, spelling and punctuation.
You may often be asked in college to take essay exams.
In certain ways, the same principles for writing good out-of-class essays apply to writing good in-class essays as well.
Prewriting is just getting all of your ideas on paper. Make sure the reader knows how it supports the topic sentence. The short recap is probably a sentence explaining what this paragraph said. Once again you'll need to smoothly flow into the next paragraph.
There are many ways, and maybe prewriting deserves an instructable of its own.