Jennifer,
You can make multiple attempts of the same exam, so you can always eventually get to the questions you didn't finish the first time. However, for every attempt, you cannot pause the exam, so you will be assigned a grade based on how many questions you did get through. If you don't have the time for the whole thing, I recommend doing quizzes instead which have similar questions and can be tailored to whatever knowledge area you are weak in.
- Steven