I spent a lot of money on a Finance degree. Then I got my first job and they gave me one of these calculators along with the manual [1]. I then realized that everything I had learned was distilled down to this little booklet. It was a shocking realization. I still have and use it. It's a good example of great design.

[1] http://h20331.www2.hp.com/Hpsub/cache/299940-0-0-225-121.htm...