On this forum a great deal of advice is available on which books are best used to teach oneself almost any subject. Everyone also suggests doing the problems in the book. My question is how many problems do you suggest doing? On this boards recommendation I taught myself most of Spivaks Calculus. I did maybe 75% of the problems in the text. I really enjoyed the text but doing so many problems took an extremely long time (I found many of the problems quite difficult. In general I would like to teach myself more subjects (right now Algebra from Artin and ODE's from V.I. Arnold). However I have nowhere near enough time to do every question in those books even I don't sleep (and I plan on sleeping). In general I guess my question is when you learned a subject (either on your own or in class) and felt like you learned it well how many problems did you do? I have tried to find good problem sets on MIT opencourseware (they have one for Artin) but it seems like the courses only list around 10 problem sets of 3-6 questions per semester. This seems far too few (even though the questions do appear difficult.) Thanks for any advice.