Hi, Lamar!

The Khan Academy mastery system is designed to measure long-term learning. As such, the Mastery challenge is something that makes sure you have learned and retained your math skills.

That’s why Missions have a wait time between mastery challenges. The main reason for the wait time in the Mastery System is to test knowledge retention over longer periods. You can read more about "Spaced Repetition" in the following Wikipedia article Spaced Repetition.

This doesn't mean you can't keep learning, though! You can keep practicing even after the mastery challenge is complete. By the time you reach Mastery, it will be easier for you to retain what you learned.