How long will it take you to solve this problem? The goal is to find the values of x1, x2, x3, x4. The rule is, input = output. Time yourself to see if you can beat the record. GO.
As another quick thought, consider how fast things go obsolete. Tonight my Marine Bio notes are worth their weight in gold to me; tomorrow after 12:30 I’ll want them only to burn. Same with all class notes in subjects not pertaining to my major (read: all my classes this semester). Crazy.
– KF –