Randomizing a mastery test with multiple questions

Hi.  I have my mastery test in a question bank.  It consists of 25 questions. I only want the mastery test to include 20 questions but I want those 20 questions randomly selected from the full set of 25.  In addition, I am allowing my learners to retake the quiz if they fail so I want these same 25 questions randomized for any retests - potentially giving them questions that they haven't seen before.  

But I don't see any option to only show 20 questions.  I can select 20 of my 25 questions - which leaves five questions wasted and I want to use all 25.  Am I missing something?  I've looked in the tutorials and I can't figure out how to do this.  Help!  Thank you!  

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