Can't find the exact policy reference, but I remember that if you don't understand Jeff's problems(in HW or exams), you could write "I don't know" and get 25% of the grades for the problems

not exact reference towards the policy: "In the table below, green scores are above 95% and red scores are below 25% (equivalent to "I don't know" on every page); those outliers were excluded when computing statistics and cutoffs." - https://courses.engr.illinois.edu/cs473/fa2012/