How do I select only MCQs that I got incorrect or haven't answered?

The MCQs are automatically chosen using the priority of 1) not seen 2) not yet correct and then 3) already answered correctly.

For example, let's assume that a topic has 30 questions, and you have already attempted 20 questions and got 15 of them correct. 

If you ask for 12 questions, you will get the 10 questions you have not seen plus 2 of the questions that you haven't yet answered correctly.