Challenges are simulated Quant sections of the GMAT, with fairly "challenging" quesitons, created by senior members of gmatclub. Although they are much more difficult that real GMAT problems, they are said to help a lot in improving one's math concepts. You can find them in the Challenges link on the main page. Click on archives and you will be able to access challenge #25. For access to the older ones, you will have to purchase them.