One of the goals of challenges is to see whether you can answer problems correctly without being told which skills they cover. For this reason, you can’t choose skills for your challenges. However, you can increase the likelihood that a certain skill will appear in your mastery challenges by adding it to your dashboard and then practicing it. Skills you’ve practiced are prioritized in mastery challenges. So are skills you have to recommend to yourself, or that your coach has recommended to you.
The Mastery System
What is the mastery system?
What do Khan Academy’s mastery levels mean?
How does Khan Academy measure my math knowledge?
How can I see which skills I’ve mastered?
How does the mastery system help Khan Academy personalize my learning experience?