You don't need a calculator to do simple math! Learn a few tricks, and with a little practice, you'll be surprised how much arithmetic you can do in your head.
You can read the full text of this hack at oreilly.com.
Concrete ideas to modify and expand this hack go here.
Freeform discussion of this hack goes here. Test.
Links to relevant websites go here.
Errata go here.
MarkPurtill?
This is a page for a hack from the book MindPerformanceHacks by Ron Hale-Evans.