26. The system of claim 25 , wherein the means for generating a series of mathematical problems comprises means for randomly selecting types of mathematical problems from the plurality of mathematical concept categories.
26. The system of claim 25 , wherein the means for generating a series of mathematical problems comprises means for randomly selecting types of mathematical problems from the plurality of mathematical concept categories. 27. The system of claim 26 , wherein the means for adjusting the series of mathematical problems comprises means for varying selection probabilities for the plurality of concept categories based on the measure of user mastery for each of the plurality of mathematical concept categories.