Hi Arnaud,

You're right - Math by Subject > Statistics & Probability doesn't link to a regular mission with Mastery Challenges. I've noticed this, too!

But Math by Grade > High School does point to a Probability mission with Mastery Challenges. Have you seen it?

https://www.khanacademy.org/mission/probability

I have the same complaint. The high school probability mission is too light. Please open a mastery challenge for Statistics and Probability!

 Also, there are two different arithmetic pages - arithmetic, and arithmetic - all content. There is no mastery challenge for the latter which has additional content such as significant figures. Why isn't this content part of the main arithmetic mastery challenge?