Page Not Found (GRE Math Review)

I have been using the GRE Math Review guide provided by Khan Academy through ETS. 

https://www.ets.org/gre/revised_general/prepare/quantitative_reasoning/khan_academy/

A month ago, all the links directing me to practice material worked but now about half (if not more) of the links do not work at all. For example, all the links under Algebra: 2.8 Coordinate Geometry do not work for me and state "Oops! Sorry, the page you were trying to load doesn't exist."

I have tried refreshing the page and logging and out of my account.

What can I do?